Hi idlewild,
Hopefully you’ve had the chance to read the tutorial post which has all the basic information, steps, and guidelines you will need to make the best possible choice.
Yes … there are many local and online options that may all be good choices for you but the goal of course is to decide on which ones are the “best for you” out of all the ones that are available for you to consider. If you follow the steps of the tutorial post then you will end up with some finalists that would all be good choices based on the criteria that are most important to you rather than making a decision based more on marketing information than anything else or without having a frame of reference to compare them. That’s not to say that the Casper wouldn’t be a good choice or even the “best” choice for you … only that you won’t know until you have some other qood quality/value mattresses to compare it with so you have a wider frame of reference. There is more about the most important parts of the “value” of a mattress purchase in post #13 here that may be helpful. “Hip” will have little to do with how well you sleep. 
Casper is an “all or nothing” choice because they only have one mattress. If it’s a better match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences), the quality and durability of the materials, and all the other parts of your personal value equation that are important to you than any other mattress that you are considering then this wouldn’t matter but any single mattress will only be the “best” choice for a small percentage of the population, a “good” choice for a slightly larger percentage, and an “OK” choice for a larger percentage yet so it depends on whether you are confident that it’s “the best” or a “good” or an “OK” choice for you compared to any other mattresses you are considering.
The good news is that they have a very good return policy so even though they don’t have any other options available … you would have the chance to try it and decide on whether you are in the percentage where it would be the “best” choice or perhaps at least a “better” choice for you compared to your other finalists. If it’s not then you can return it and choose another mattress.
PPP is always about a sleeping system and it would make little difference whether it’s a mattress or a mattress/topper combination if you are in good alignment and have good pressure relief in all your sleeping positions, and it also is a good match for all your other personal preferences in terms of how it “feels” and “sleeps”. In other words there is really no such thing as “defeating the purpose of PPP”. A topper also has the advantage of being replaceable without having to replace the entire mattress if your needs or preferences change down the road or if it softens or breaks down before the rest of the mattress (the top layers of a mattress are generally the weakest link of any sleeping system whether they are inside the mattress cover or outside it).
You can see my comments about this in both of my replies in the other topic you posted in here. I would be concerned if this had happened to me as well and something similar actually did with a memory foam topper I purchased at one time … (see here) although I don’t know why I was sensitive to that particular topper and not to other types of memory foam that I have had no issues with.
The materials in the Alexis are higher quality, more durable, and more costly materials than the materials in the Casper mattress (they use blended Talalay latex which is a more costly material than synthetic latex and both are more durable than 4 lb memory foam). As you mentioned it uses a higher quality/density and more durable polyfoam in the base layer as well. Although the base layer isn’t normally the weakest link in a mattress in terms of durability … it can still make some difference, especially for those that are in higher weight ranges. Most “should and shouldn’t” questions are more about your own personal preferences than anything else and they depend on whether they would be a good thing for you. There is more about the different types and blends of latex in post #6 here and there is also more about all the variables that can affect durability and the useful life of a mattress relative to each person in post #4 here and the posts it links to.
If for example you know that you prefer sleeping on talalay latex compared to other types of foam materials or combinations then if you have a latex mattress that needs to be fine tuned for firmness/softness but you know that it’s just a matter of changing the firmness level and not a matter of changing the material to something else because you know you prefer Talalay latex then having the ability to exchange layers for a different firmness gives you the ability to make firmness changes to the mattress without having to start all over again and find another mattress that uses the same materials in a different firmness level. Having one layer that you can exchange can often be all you need but in the case of the Alexis you can rearrange the layers or change either the top layer (which is more about pressure relief) or the middle layer (which can have some effect on both pressure relief and support as well). In other words … it would be a matter of whether you preferred to be able to return a mattress and find another one that was similar in a different firmness level or you would prefer the ability to fine tune it. Component layers also give you the ability (like a topper) to replace individual layers down the road instead of replacing the complete mattress. Which one was best for you would depend on the parts of your personal value equation that are most important to you. If a mattress was perfect from the start then it wouldn’t matter much (outside of being able to replace individual layers down the road) but if you sleep on the mattress and you discovered that it wasn’t ideal for you then it’s a great option to have available. It would be something like having the ability to choose between several different mattresses that were the same design and used the same materials but have different firmness levels based on your actual sleeping experience rather than your “in store” testing.
The same value guidelines would apply here. The most important part would be how this mattress compared in terms of PPP and the materials you prefer to sleep on. The second most important part would be to know the type and quality of all the materials in the mattress (see this article) so you can identify any weak links in the mattress. In this case I would be very cautious about the 2 1/2 inches of polyfoam and the fiber in the top layers of this mattress and I would make sure that you find out the density because it’s very likely that this would be a weak link in the mattress in terms of durability (the guidelines I would use are “no more than about an inch or so” of lower quality/density or unknown quality materials in a mattress). Buying a mattress where there is more than about an inch or so of lower quality or questionable materials can be very risky because lower quality materials can soften quickly and you can lose the comfort and/or support that was the reason you purchased the mattress in the first place much more quickly than a mattress that uses higher quality materials … and the loss of comfort and support isn’t covered by a warranty (see post #174 here).
While it’s certainly a “better than average” choice compared to most mainstream mattresses that most consumers would otherwise end up purchasing … a mattress that is marketed as “one size fits all” is mostly geared to consumers that are in a hurry and don’t have the time or wish to make careful comparisons based on all the criteria that may be most important to them or those where finding the “best” match for them may not be as important as finding something that is “good enough”. It could still end up being the “best choice” for you but for most people the attraction of this type of mattress would be more about the simplicity of shopping for a mattress that was “probably good enough” than it would be about finding the best for them.
I can’t tell you which mattress would be best for you and only you can answer these types of questions based on all the criteria that are most important to you but once you have read the tutorial post and the links in this reply you will be in a much better position to know “how” to choose between any of your finalists. If you can’t identify a reason that any particular mattress is “worth” $1000 more then for you it probably wouldn’t be. You have many good options available to you either locally (see post #2 here) or online (there are links to several lists of the better online options I’m aware of in the tutorial post) but you will always be the only one that can make a final choice between them. Hopefully this reply (which included many of the links in the tutorial post just to give you a “sample”) will be helpful
Phoenix