That's a terrible idea. Dungeons need to give better exp and better rewards. There's no reason that they couldn't add Tombstones of Philosophy and seals to lower level dungeons for anyone with a level 50 job. It'd encourage people to actually run them. Your idea just means that people will just find a more efficient way like spamming leves or spam killing mob camps. As it stands for the effort and time put into dungeons they just aren't worth doing until endgame or for a goal.