While I agree that web based email clients may be the way of the future, it will be a long time before messaging services provided by sites like MySpace or Facebook replace traditional email services. The main problem with these services is their inability to relay messages to users of different sites.
From a technical POV, the move to web based email services could actually be considered a good thing since it abstracts the underlying system from the user. That is to say, users need not know how it works or how to make it work, only that it does what they expect it to do. If web based email were to completely replace desktop email clients, we would be free to modify and correct the shortfalls of the current system without having to conform to protocols imposed by existing desktop based email clients.