the whole point of having the items drop in the low dungeon is to have high level people lvl sync so those exping can get a party to do the dungeons. It would take much longer que times if only those exping were in the party.
It does not help. I waited 30 minutes to get into that dungeon, @5 minutes, they kicked me. And after I had to wait another hour to get back in.

The problem lies in the fact that lv 50s prioritize themselves and give no concern to anyone possibly leveling still.
This is a fact. Peoples posts here prove it.

Running dungeons isn't the only way to get exp.
Yes there is, but not everyone like to do fates and or Leve's.

Or they could make it so you only get exp in a dungeon if you're in a preformed party, that would have the same effect.
Would be a bad idea to do. Reason being: New players don't know the system or have friends to help. So you'd be screwing they and they'd end up quitting.