WoW has two strong components that do wonders for player retention, IMO:
-sense of progression that you can't let go of easily since you've invested so much
-familiarity with players and social bonds
Letting players solo all max level content may be a highly requested change to the game but in the end you'll ruin yourself so long as you rely on player retention. You really don't want WoW to feel like a game you can pick up and drop, you want them to feel like that can't leave, you're trying to sell a drug here, don't cut down it's potency so players can get on with their daily chores, get them hooked!