For the price point, IKEA mattresses (both hybrid and foam) are worth it. Same goes for mattresses sold by Costco/Sams Club.

A lot of this enthusiasm about mattresses comes from being young. When your back is still indestructible, nearly any mattress(all kinds of foam, coils, hybrid, innerspring) feels fantastic. 20% of mattresses are returned for comfort reasons; that's how online marketing companies disguised as mattress companies have won over the traditional brick and mortar companies.

Lots of people complain about the invisible sagging after a few years of usage. For warranty purposes, 1.5" visible sagging is needed. Even latex foam sags too, but it sags slower than PU foams. Even Tempurpedic has cheapened their foam, thereby cashing on its brand name.

High density foam lasts longer. However, 99% of mattress makers don't list ILD and densities of their PU and viscoelastic PU foams. That's why the market is flooded with cheap mattresses that have invisible sag after a couple of years.

Same goes with coils: thin wire, reducing the wire in each pocket, stretching the wire, carbon content, how wires are cleaned, etc--all these factors matter.

Yes, there is advancement in the knowledge of materials and foams. However, industry has started cutting corners for a short term profit. If you make a mattress that lasts 10 years at least, who can you sell mattresses to then? That's why cheap low density foams, cheap coils dominate the industry.