Hi wvk,
Thanks for taking the time to share your “hotel mattress” experience … I appreciate it 
Unfortunately your experience is not unusual for those that buy a hotel mattress which are normally just a lower quality and less durable version of similar major brand mattresses made for consumers (which I would avoid as well because they also tend to use lower quality and less durable materials than many of their smaller competitors). You can read more about hotel mattresses in post #3 here.
While a warranty is important to guard against manufacturing defects in the materials or components inside a mattress that can happen on rare occasions … as you discovered mattress warranties only cover manufacturing defects and they don’t cover the gradual (or more rapid in the case of lower quality comfort layers) loss of comfort and support that comes from foam softening that is the main reason that most people will need to replace their mattress. In other words warranties have little to do with the durability or useful life of a mattress or how long it may be until you need to replace a mattress. If there is an actual defect in the materials it will usually show up early in the life of the mattress but knowing the quality and durability of the materials in your mattress is always a much more reliable way to assess the durability and useful life of a mattress than the length of a warranty. There is more about mattress warranties in post #174 here.
I’m glad to see that your topper has helped.
While there is always some risk and uncertainty involved in adding a topper if you haven’t tested the combination in person because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress … if a mattress/topper combination is a good “match” for you in terms of "comfort and PPP (Posture and alignment, Pressure relief, and your Personal preferences) then it also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is very common because the upper layers of a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time and a topper can also help extend the useful life of the mattress underneath it as well.
Phoenix