Hi flieder,
I would keep in mind that a mattress is designed for sleeping not for sitting on the edge and that with the more concentrated weight of sitting on the edge with more point elastic materials like latex that this isn’t uncommon. Zip covers are normally designed to be tight fitting when they are zipped around the latex but they can fold if you sit on the edge of the mattress … particularly with softer or thicker layers. This is just part of the design of this type of mattress and certainly isn’t a “quality issue”.
The price of a mattress has little to do with how it feels which is very subjective. You could purchase a $10,000 mattress for example that would feel horrible and be completely unsuitable for some people to sleep on and yet would be the “perfect” design and for others. The cost of a mattress has more to do with the cost and quality of the materials inside it (more durable materials like latex or quilting materials like wool tend to be more costly) and the labor involved in manufacturing it than with how it feels.
No I haven’t seen one in person but I have talked with Scott about his cover a fair bit. It’s a high quality cover that he has custom built for their mattresses and he is very happy with it (he has used many covers over the years and he particularly likes this one).
You can see a little more about the pros and cons of a zip cover and replaceable layers vs a finished cover with glued layers in post #2 here and post #15 here.
I would also keep in mind that if you are hoping to keep a mattress for 20 years that your own needs and preferences may change by that time even if the materials haven’t lost their comfort and/or support and this may be the limiting factor in the useful life of a mattress. It’s more realistic to expect that you will sleep well on a high quality mattress that is a good match for you in terms of PPP for about 10 years and I would consider any time after this to be “bonus time”. With component mattresses you can replace individual layers if one of your layers softens or breaks down faster than the others (usually the top layers) or if your needs and preferences in a mattress change over time.
Phoenix