I think Emma is the least likely to stay - she's so cautious about it and the WB president Alan Horn needed to convince her to stay for GoF, he made a personal appeal to her.

Most likely to stay is Rupert, ironically. He's the oldest but the most keen - same for Tom, he's the eldest and most keen to stay after Rupert.

Responses to 'will you stay':

Rupert: "Yeah, I'd love to do all the films!!!"
Dan: "We'll see, one film at a time..."
Emma: "I really don't know ummm I guess I'll wait and see..."

That's sort of their typical responses to these sorts of questions.

Tom Felton has even said he'll delay going to fhising college to finish the films!! How committed is that!!! (that was in the same K-Zone article where the WB appeal to Emma was mentioned.)