I think you're unaware due to where you are in the story, but the amount of dungeons not connected that the story has reduced over time to the point where there are none. Removing dungeons that can spoil story stuff includes the majority of dungeons from Stormblood and every dungeon from both Shadowbringers and Endwalker. The last two expansions have only two dungeons each that are not part of the msq but all of those four dungeons build off very spoilery main scenario concepts.
For you this would unlock maybe two or three dungeons, at best and that's it. They've cut down heavily on the number of side dungeons over they make over the years.
There is also the other issue, others have mentioned of lower level jobs struggling in higher level dungeons without their full kit. Just unlocking those abilities temporarily for the higher level dungeons would put a lot on those players. It'd honestly be worse for them than a freshly level skipped player.
