That doesn't work. In games that do this, all that really results in are mobs with higher HP, or normal mobs swapped out with mobs that have more HP and have a more complicated AI.
Eg take sastasha, 4 man. Make a "Savage" version that is now an 8-man, and every trash mob has as much HP as the boss fight, and the boss fight's have insta-wipe level damage. The dungeon is otherwise the same map, and exhausting to do since now instead of a trash mob pack taking 30 seconds, it takes minutes.
The reality is that in other MMORPG's that do this, you end up in this upside down situation where players just want the reward, so they will instead (pay someone to) get carried through it just to get to the reward at the end. Thus back to the problem we've had since 2.x, with very few people actually legitimately "clearing" content when it's new, and instead waiting for it to be de-facto nerfed by the next expansion before trying it to avoid the P2W extortion racket.
Don't underestimate players desire to take shortcuts and want instant-gratification. Ever ask yourself why players would keep finding ways to keep doing Labyrinth of the Ancients from the roulette for tomes/cracked-materia-items rather than any of the harder raids? Because players would find ways to manipulate the roulette, first by wearing lower level gear, and now apparently by having someone who hasn't unlocked anything but Labyrinth to be in their party when they queue for the roulette. That loophole needs to be fixed.
My preference really is that if players really want harder content that is still accessible is to have variations on the "Field operations" applied to dungeons and trials, where you earn mettle by clearing it on a per-player score (eg being carried won't earn you points,) and your gear has specific strengths/weaknesses that need to be rotated on a per-fight basis (so you have to actually pay attention.) But I don't see this happening since the Field operations aren't very popular since they've been designed as time-sinks. All "special weapon" grinds are.
Difficulty != Timesink.