The problem is that, as things stand, very few higher-level players are willing to go back and do low-level dungeons if they don't have to (either leveling a new class or because they need a drop). I don't know anyone who does LLR unless they need it... why? Because it's extremely boring.

The more players who hit 50 and stop doing low-level content, the fewer players there will be to help newbies out and the longer their queue times will be. You have a problem when most veteran players don't want to touch low-level content.