When it comes to rewards, endgame is where the prize is. Why does it matter if a high level can go into a mid-level dungeon. The drops won't be geared towards endgame anyway. If you want to stay true as a gamer and go for the challenge, just don't take a high level player in with you...
The high level dungeon has a cap, and that's 50 the highest level you can gain, the challenge will always remain there, and likely the true prize and drops as well.