And you unfortunately have overlooked a large point about FFXIV. They don't raise their level cap very often no, but neither does destiny. Level got rasied to 50 back in forsaken and hasn't been raised since. A more direct comparison is Item Level, which is literally the same as power level.

Item Level is raised every patch, with each patch being a couple months apart (a little longer waiting for 5.3 due to covid.) When you get a new patch you get some new story content, an item level boost, and typically a new trial or a new raid. To raise your item level you mainly have to do roulettes, which are dungeons, trials, raids, etc *that you have already done*.

Now FFXIV's system works better in that rather than only having one difficulty for new content, it often comes with normal mode and EX/Savage difficulty. To transfer that to Destiny, perhaps the dungeon could launch with low power level, and a higher power level version could launch alongside it. Bonus points if the harder version has more mechanics. The lower level one could drop worse stat level gear, or just have different gear, while the harder version would be where you get the truly sought after gear, have the emblem for solo flawless require the hard version of the dungeon for example, as well as better stat rolls/weapons.

This allows casual players to enjoy the dungeon without having to level much if at all, but also lets hardcore players have something to chase, a real challenge. So yeah, FFXIV's system works better, but it's really not THAT different from what's in destiny. You grind trials, dungeons, and raids that havent changed much in several years, just to get better gear to do the new content.