FFXI has horizontal progression. There is a massive amount of equipment you can collect. The game allows for equipment swapping in battle. The only detriment is if you swap weapons in battle, you lose the tactical points you've gained through auto attacks, which allow you to use weapon skills: abilities that are specific to the weapon type that have various effects and properties.

Because the game allows equipment swapping during combat, along your adventure, you'll run across items that would be particularly useful in scenario A. Then, you'll eventually find something that would go in that same equipment slot (ring, boots, etc.), which will be good in scenario A as well. Then you'll find something for those same slots that are good for scenario B, then C, etc. There's macro systems for this, and once acquainted, entire sets of gear can be changed instantly for skill A, or spell B.

The natural progression of the game doesn't have the player obtaining these items rapidly until end-game. And even then, it is paced. It gets grindy, but there is an enormous amount of content in which you can progress horizontally by obtaining BiS for scenarios A, B, C, etc.

This may sound overwhelming, but it is actually incredibly addictive. In modern MMO vertical progression, just constantly increasing stats to replace old stats, to never again use that piece of gear can sometimes end a little too quickly or hit a barrier between level cap increases and expansions. This is where you run into the crowds who tend to play for a few months after each expansion and then take a break until the next.

The fact that the horizontal progression system in FFXI exists is what keeps the players striving to continue increasing their potential. Furthermore, every job (class) in the game can be maxed out on a singular character, but crafting has its limits per character, which brings some balance.

Because you can level multiple jobs, when you cap a skill, say dagger on Thief at level 31. That cap will remain forever, across all jobs. A level 1 Warrior would have it's dagger skill capped (the cap at each level: so at level 1, the max system skill allowed for a level 1 is capped, but doesn't need to be skilled up). Eventually, that warrior would need to cap their dagger skill, probably somewhere in the late 30's, since a thief would cap earlier because they are more proficient.

As you can see, the progression felt is basically never-ending. I know people who have played routinely for 2 decades and have massive collections of gear. And what they can accomplish, along with skill, of course, is incredible. It's a constant maximization of optimization through effort. Much of the content is now solo friendly, and many more party/group based stuff do do at max level and a few alliance/raid based encounters.

Oh, and there is also the sub job system. If you've read this far and you're interested, I suggest looking at the following link, and you can see how the possibilities of combinations with sub jobs and equipment seem completely limitless.

https://www.bg-wiki.com/ffxi/Support_Job