This is a problem I see too often with MMO games and wonder why more developers don't focus on fixing it. Low lvl content becomes a wasteland with very few or no players in it after a game has been out for awhile. They may as well delete the low lvl zones, especially when games are starting to offer catchup boosts with their new expansions or w/e.
I know they can't force players to go to these zones, but the issue is that they don't have a reason to. This isn't an issue in all MMO games but it is in a good portion of them. Seems like a waste of time to make all that content when it's just left by the wayside. I'd like to see future developers do something about this, but current developers could too. Make your worlds alive and not just a means to an end!