And perhaps that is where the problem lies. Areas that are partied by level 10~20 players have dangers that are relative to level 10~20. Which means monsters around that level range. Which means it's only going to be a bother to players at about that level range and a level 50 player will run by like it was nothing, on virtue he's a level 50 player.

I'm curious. How the hell would a L15 area pose a death threat to a level 50 player without making it completely unplayable for a L15 player?

And why in heaven's name do you think a cavefull of aggro monsters five levels above you won't get in your way regardless of your level? Speciallyat lower levels where you have no survival tools avaiable?