Counter argument:

One of two things will happen if the level cap stays fixed. We'll either see new gear come out as upgrades or we'll see new gear come out as side grades.

Gear that comes out as upgrades has almost the same effect as increasing the level cap, in that old gear becomes obsolete. However, it's not usually as bad as a level cap increase.

Gear that comes out as sidegrades becomes boring fast, runs the risk of becoming obsolete on release, or being subpar. FFXI's Sea, COP dynamis, and Hydra HNM didn't see much action because the effort was too great and the gear they offered sucked.