Actually, after you reach max Level, you open up the ability to explore the other alliances' zones. Which means that in least in some form there will be reason to go to lower-level zones post-max.
You could always go to a weapon and armour that is low level still and play, sure you are going to be level 50 in a level 10 zone for example but your skill with the weapons and armour will be pants.
I have not heard of downscaling in the game although it would certainly add to it if your friends have out leveled you and wish to run a dungeon with you.
Once you reach level 50 the other faction PvE zones open up and you can pick one and then the other, this also opens up harder dungeons and new areas in the ones you have done previously.