Aha, I didn't understand at all before your edit (no offense :P), and I think you have a good point, but it is actually on the "pro" side for approaches like Winer's: the hypothetical "Twitter-like ecosystem" is targeting now, just like Twitter (less the realtime-related problems, sadly), but is set up to continue targeting "now" once nobody uses Twitter, even during the transition period!
Isn't this exactly the problem? Keeping a service alive in perpetuity is missing the point because it's the communication that's important, not the service upon which the communication flows... Human communication is inherently temporally limited. If you ignore that, you're not gonna be heard now, which makes you even less likely to be heard in the future.
EDIT: I guess I'm just saying: nobody reads old messages on this sort of service. If you're not targeting "now", you're starting off with an immense disability.