capping levels at 100 is fine, but staggering skill evolution is not, fights are going to get harder and more challenging that's the nature of video games in general, so skills have to be raised and developed over time, and I don't think that has anything to do with leveling that can be a simple matter of devs seeing what works and what doesn't and altering that that doesn't, in game wise we could go back to job quest where the point of it is to gain a deeper understanding of the job and obtain a new ability either self taught or learned from a master or legendary member of the job.

As far as making old content trivialized I have to disagree, the level caps on instances prevents that from happening as well as equipment caps, even at lv 90s being brought back to some level 50 content can make things challenging as you have less of your kit to use or unevolved abilities that don't offer the comfortable bonuses you were once used to. Was in Vigil Hard the other day as a healer and I swear the tank needed me to babysit him through the whole thing, I almost never got a chance to dps cause I had to keep my eyes on the party HP because they were getting spanked so hard, I wanted to say they were under armed but I took a look at the tanks equipment and it was indeed level 50 gear so he wasn't underprepared, and was a warrior. It was quite eye-opening.

I do like the idea of level capping us at lv 100, and just designing newer expansions to evolve our current skills into newer stronger ones, majorly reworking jobs that haven't been performing within the original design of the job. creating new jobs that start at lv 1 and just give us something to work toward which would have us revisiting older content or doing deep dungeons longer who knows.

My addition would be an ascension Tier, where all jobs that are lv 100 ascend returning to what could be described as level 1 but isn't. For content level 100+ a new job will ascend and begin leveling a new in a sub-tier called the ascended, where you will gain faux levels to a certain cap that will grant the evolved skills I mentioned before as you progress, your hp and mp and all that stuff stays the same prior to the new content but what skills you have change. This keeps you looking forward to new content, gives the devs time to develop each job toward this new far-flung goal of evolution, Maybe Paladin changes to Holy Knight with certain job actions becoming more flashier, Dark Knight could become Dark Lord, so on and so forth.