We'll see how it turns out, but XI's lateral shit was a by-product of the gear-swapping. And I really hated that I had to do ALL THIS STUFF to put together a decent set of armor for one spell, etc.
So having progression means that you might actually get to FINISH content and move onto something new, rather than doing the same thing for years on end.
I think maximizing multiple gear sets always felt like squeezing out smaller efficiencies (relatively speaking) here and there. Once you had a good general set for 75, you were good to go on most content and then spent most of your time increasing damage or speed over several situations (especially once you had a slightly less generalized "Weapon skill" set mostly completed). Only downright stupid things were huge upgrades (and often enough you could contribute that to the game becoming downright stupid with zerg rushes). Aside from half of the elemental staves (which isn't actual endgame gear), it wasn't like you weren't decent if you didn't have specialized gear for everything. Not quite the case if you are whole "tiers" behind. Gear checks were less likely to be a thing in FFXI (and when they were it was sometimes a player created one due to trying to down the boss in 2 minutes).
I like the idea of content never being "finished" or at least only being slightly redundant at worst. That's not to say I'm pro "bad drop rates", which is partially the reason why you were never done with anything. The idea that FFXI's endgame consisting of a dozen different events (although not of equal importance) was always a positive in my book. Variety is the spice of life (and to a lesser extent you may be able to apply this to gear). It doesn't quite work out as one would hope thanks to drop rates and other unnecessary barriers/time eaters combined with people wanting to be slaves and not adventurers in MMOs, but it is way more appealing to me than being funneled down a path, with finished content being trivialized (more so with expansions upon expansions).
Both methods are designed to keep the player playing for longer than they have good reason to, but my preference lies in the one where you always have something to do or at least something different to do. I'm biased because I rather do everything than do the things which are deemed most "rewarding". I'm also biased because there is less pressure to do endgame events if there are fewer gear checks (upon gear checks).
I will say though that without gear switching (and without elemental specialization on top of that), you don't have a lot of room for specialized gear. You are basically left with trying to obtain the single best overall piece of gear and maybe one or two alternatives for extremely common situations at the very most (probably high eva and low eva lol). So what room is there for multiple events around the same "tier"? What about crafting vs end-game events? Never felt like XIV had to have gear switching, but I can see how getting rid of it can negatively affect content.