I think everyone's vastly underestimating the amount of play value in two levels.
According to the DMG, it takes about eight encounters to level up in 4e. Each encounter takes about one hour to play, and each game session has about an hour of ramp-up and wrap-up time. (This matches my experiences playing Scales of War very closely--playing once a week for three hours each, it took us about a month to finish a level.)
So the red box set has 16 hours of encounter gameplay in it, plus an hour per session, plus some solo stuff, plus dice, plus tokens, plus power cards. That's a decent amount of gaming for $20, and easily beats video games or movies in terms of entertainment per dollar. It's certainly not "play it once or twice."
Yes I know that, I was somewhat voluntarily understating the facts.
That's because IMHO I still see the new Red Box as a teaser for the complete game. I'm not sure "replayability" is something the authors are looking for or even wishing for.
For the sake of selling the game, the earlier the new players are hooked, the better. In this digital era, if kids aren't hooked by level 1, they will look for something else. I see even level 2 as a sort of (welcome) bonus.