bed nest culprit

I think my bed may be teetering on expiration.
From the "they just don't seem to make 'em like they used to" department, .. I used to have a tremendously good bed (my parent's old custom made mattress/box spring set), but it was extremely heavy and it did not come along in my last move.
I did keep my mom's more recent bed (smaller, lighter, but more modern factory assembly line construction), and alas, it seems to be sagging quite a lot on one side. At least I hope that was the culprit of me waking up in pinched-nerve agony twice last week ;P

Following that theory, I "turned the mattress" as they say one should do, and am trying the non-concave side now.. so far so good, but I don't know how much longer this thing will endure.

I did invest in a Purple mattress some years ago, and it was a delight, but it ALSO developed a cavernous curve just about a year into owning it, so they don't seem to be very sturdy... thus probably will avoid that if/when I do finally need a new one.

I wonder how the good Casper mattresses are...