Hi there -- Thanks for your question. A couple things to address here:

1) It is not necessarily a myth that over its lifetime, your mattress will inevitably accumulate some amount of dust, dust mites, and other icky things. It is, after all, a soft upholstered object and is exposed to a significant amount of heat and moisture from your body on a daily basis. That said, the notion that these accumulations will ultimately cause your mattress to double in weight (or even increase substantially in weight) is most definitely a myth. We discuss this more here:

https://www.goodbed.com/guides/mattress-owners-manual/dust-mites/

2) As far as how they make mattresses now vs how they did in the 90's, you will indeed find that by and large, they do not make mattresses like they used to. For the most part, these changes have served to make mattresses more comfortable, more conforming (esp for side sleepers), more pressure-relieving, and to enhance them in a number of other ways. However, as it relates to their longevity specifically, these changes have generally been for the worse. Mattresses back then were much more basic, which allowed them to avoid body impressions and to maintain a more consistent feel over time. In addition, most mattresses made before 2000 were flippable, which helped to smooth out any body impressions that did occur. For these reasons, it was much easier back then to find a mattress that could serve you well for 25+ years than it is today.

I hope that's helpful.