Even putting aside the tank issues, you'd have the problem of mobs just dying way too fast. Palace of the Dead is a poor place to learn a class simply because nothing lives long enough to bring the full suite of abilities to bear. This change would make dungeons far more PotD-like. It probably wouldn't be as bad but new players would lose out on pacing.
Further you'd neuter what challenges the dungeons do have. While Satasha->Thousand Maws probably don't have enough substance to lose, beyond that it is an issue. Think on something like Sunken Temple of Qarn. This dungeon poses real challenges to a group of new players. Needing to kill bees before "Final Sting", having to deal with Doom, killing the golem core before the healer's MP pool is gone. These issues basically all vanish if max level players can vaporize the mobs with max level rotations and potency values. Stone Vigil, Brayflox Longstop, Aurum Vale. All of these have mechanics that teach good lessons that are more trivialized the higher DPS climbs. These are valuable learning experiences.
Another reason is clarity. Say a new player is trying out a few jobs at once, he gets into a Haukke Manor run on his White Mage. He's been thinking of trying out a DPS class and sees a Ninja in his party. This ninja is the same level as him: Level 30. Holy cow that guys shooting lightning bolts, summoning giant frogs, he can teleport, and he has that awesome buff that helps the tank get aggro. "~Sweet~" he says, heading to the Limsa to start a rogue. A couple days later he's 30 and excited to equip his job stone to see his cool new toys and... "Oh. Wow, those are all level 50+. How the world was that other guy using them? This is kind of unfair...". It places a burden on new players to do external research or ask exhaustive questions to be able to have a fair understanding of the context of the group content.
However if we put aside those issues another big one remains, a massive difficulty spike at 60+. Let's say we're OK with players not learning those specific lessons, and bosses going down a bit too quick. They're going to hit a wall and hard when they reach level 61+, as their party members kits start to come into alignment with the content. The effective amount of "Carry" they're starts going down at a far greater rate than any of us veteran players were exposed to. Not that dungeons are the most challenging content but they will certainly seem that way next to the previous 60 levels of steamroll carries.
Besides, the purpose of the roulette system is not to be terribly exciting max level content. Leveling roulette is not around for the benefit for us. It exists for the newbies. It's low level basic content of course it is bland for a veteran. The tomestone rewards and exp for higher level characters are bribes to get us to do content that wouldn't otherwise be engaging to us. If it was supposed to be otherwise they'd get rid of the daily limit, tune the rewards a bit and slap every 70 ability they could inside. Instead the system exists to preserve the low level content in as close to an "Original State" as it reasonably can for new players. Once you make changes to make it compelling for high-level players you totally lose purpose, hence the bribes. So long as these bribes are enough to keep people in they have no reason to change it. If the bribes start failing to be enough they'll either increase them, or change the content the bare minimum away from the Original State to get people in.
They know it is dull, it is supposed to be dull.