BIB 1: Its adding convolution for the sakes of convolution, and the levelling system isn't just for gating story content. It adds a sense of flow and progress marking as you progress through the MSQ, its used for expanding jobs at set points (new skills and abilities, job gauge expansions), power increments, and for gearing thresholds.
BIB 2:
Endgame is only endgame during the current expansion, when a new expansion releases we get a new endgame, the jobs have evolved, the mechanics have evolved and the endgame becomes more difficult. If reaching Lv100 gives you endgame access now and going forward, even if we are 4 expansions down the line, this starts to make little sense and just sounds like a way to avoid having to level jobs. Unless the endgame for new expansions will require master levels in which you've replaced an existing system for a new system that does exactly the same thing, ergo is pointless.
Anyway, the question shouldn't be how to change the levelling progression system at 100 but why change the system at all? Just keep the level cap increasing with each expansion, add new abilities and expand the job gauge as you go, basically as we do now. If we get button bloat, adjust the abilities in some fashion - something like I explained above.
Edit:
If your issue with increasing the level cap is that you don't want to level multiple jobs past 100 and just want to do future content on the job you've got. Then, for one, that wouldn't work anyway because the new endgame at each expansion needs to have a power increment, and for two just comes across as idle. FFXIV is one of the lowest effort games out there for levelling - its faster than many single player RPG's.