I disagree with your entire premise, but this part of your post bothered me a bit. As someone who has done the gear grind several times in those "MMOs that raise the level cap", I can tell you that for every person who cried about their phat lootz becoming collectors' items, you also had people who simply shrugged their shoulders and got back into the grind because they wanted to see the content.
Raising the level cap means that you have more opportunities to not only add content and add gear, but also further develop the way your character classes play, or as companies like Blizzard have demonstrated, make really big changes to classes at a core level that would otherwise not be possible.
From a gear and itemization perspectives, the lv75 cap in XI being in place for so many years gave us a lousy selection of sidegrades that had to be as such as to do exactly what you wanted; not obsolete older content. Problem is, i'd rather just increase the power of the players over just sticking them with lousy sidegrades for several years. It detracts from the proverbial carrot on a stick that gear is supposed to be when in context of MMORPGs.
That there should be content in the game that lasts a decent amount of time, sure. That they decide to not increase the level cap is folly and detrimental to more than just the content it risks making obsolete.