I guess my only problem with that is, don't we already have like 30 dungeons that fill the same purpose?
No, we don't.

The concept of Deep Dungeons is fundamentally different because it gives people something to work toward. The leveling dungeons that are already in the game are kind of one-and-done things with no worthwhile rewards for max-level players. Deep Dungeons, on the other hand, is an expandable content system that requires repetition to get through and ultimate rewards players with incredibly useful weapons that are viable for most of endgame.

The leveling dungeons still serve a purpose for farming tomes, but Deep Dungeons offers an entirely new dynamic for players of most levels.

This COULD have been something to fill in some more medium difficulty content like people have been asking for, but it's not.
If anything, that's what Diadem should do once it's revamped, but we'll see. SE choked on it so bad the first time, I'm not exactly holding my breath for them to do a complete 180.