the game currently has, as you say, multiple options for different stats. there is nothing to stop them from making multiple gear options based on the level cap, they already do and have done that. You can still make situational gear with level cap increases, and if you do what I and some others have suggested, it even works into the design, by having upgradeable special gears.
Say for example you have the new hax drop from the level 50 capped dungeon:
Monks burning hand. Its a rare skin of weapon that actually puts a tattoo on your hands and arm in the shape of fire.
stats
141 attack
160 accuracy
180 crit rate
160 parrying
+3 evasion +3 dex
looks like best weapon in game, even though its parry is low, but then again its probably not better than a +3 hade hora for everything. (jade +3 will have higher attack and comparable ACC most likely with more parrying, but much lower crit rate no dex and no evasion bonus)
boom cap raises, and new dungeon/content adds a new level 59 knuckle.
Iron Fists looks like a metal arm sheath with some mechanical tech accents
stats
170 attack
160 acc
175 crit rate
190 parrying
+4 vit +4 str
new best in game right?
yeah but you can upgrade burning fists if get 2 more items one from finding and defeating the new hidden monster in a the new dungeon, and the other by turning in grand company points to the new NPC in the new far off area, that you have to beat the new story mission to get to, and allowing a level 55 crafter to combine them for you.
the new stats are weaker attack wise than the new weapon, but favor it in ACC and parry
the point is you can create new content, and horizontal gear choices while also increasing the cap. in fact with upgrade systems, i would by and large tend to make the upgrades generally horizontal. This will keep old content usuable, but not required, give new things to aim for, while at the same time progressing the game. I WANT new reasons to play the class i like the most, i want to get new and interesting looking gears, and get new skills. There is not as much fun in being at cap forever, with content designed to take insane amount of time to achieve anything.
Another thing to note is in this game, monsters have tiers, and learn new skills, and have advanced abilities as you get higher, any human type will have more skills at its disposal when you get higher. Cockatrices get an AoE that does multiple status effects and petrification, high level sheeps got sleep snot bubbles. Monsters so far, in this game, actually do get tougher, with more skill as you level, not just higher HP and the ability to do more damage. Also with the different SP for species types, they add more difficult species in the same level ranges that give more sp, and are worth it if your team has a lot of power.
While they could do some of these things without a cap raise, just the fact that you arent getting sp while doing most of it, would make people do it on their other jobs, and on different classes, and many would rarely get to play the classes they enjoy. For most people in ffxi, once you got a job to max merit, and exp, you rarely got to play it.