Hi orchidvoyage,
Mattress warranties cover defects in a mattress not the softening or breakdown of materials that comes from normal use and the loss of comfort or support is the main reason that a mattress would need to be replaced (and this isn’t covered by a warranty). Warranties have little to nothing to do with the useable life of a mattress and are more about marketing than anything else. When one manufacturer adds a longer warranty on their mattress then others often follow suit because they are well aware that most consumers equate the length of a warranty with the quality, value, or useable life of a mattress even though they aren’t necessarily related. You can read more about mattress warranties in post #174 here.
Phoenix