Manchester United don't need Modric that much, although they do need an advanced playmaker. I think Sneidjer's the man instead though. I can still see Modric leaving though. And Fergie isn't really a manager who spends massive every season, Man United have been linked with loads of players, and I can't see many joining, considering United's debt and Fergie's style.
Oh, and Van Der Vaart has been average this season, he started well but got worse as the season went on. Still, why would he leave? Spurs give him first team football and a chance of Champions League football. Just because they aren't in the Champions League (they aren't really expected to be) doesn't mean every decent player has to leave.
I can see some fringe players (Krancjar, etc.) and (maybe) some 'big' players (Bale, maybe Modric) leaving, but I don't think every good player is going to leave because they haven't qualified for the Champo.