Frank wrote:

Plastics tend to deform with time under stress. Also, chemically they
may oxidize and degrade. I know our old, 20 years, king sized bed
looks as unstressed as new.

Good point thanks you!

One reason why I was looking at foam and in particular
late was that I am supposed to tilt my bed so that my
head is 6 inches higher than my feet. I do this cause
I have gastric reflux or GERD at night.

With a conventional mattress.... since it has more mass
and is heavier... there is a tendency for everything to
"slide south" at night. G