How fun are those dungeons going to be for the new players when they can't hit anything because the more advanced players have already killed them with their advanced toolkits and rushed ahead while the new player is still trying to figure things out?
The leveling roulette is optional. There are plenty of other ways for higher level players to get tomestones if they find a limited toolkit in low level dungeons too frustrating. There will still be other players queueing to help the new players have a good experience.
Because those low level dungeons aren't designed around the higher level skills with the higher level potencies. Mobs would melt immediately, which isn't a good experience for the new player who needs mobs to stay alive for a bit if they're going to get the chance to get used to their own abilities.
Let's say SE had a way to allow you to keep all your abilities but their potencies would all be reduced to 1 if the dungeon level is lower than the level that the ability is learned. You'd be able to use your abilities just as you want. Would you still be happy, or would you start complaining that it's hurting your DPS?