Was Posterous created to not be sustainable? If so, then Posterous's users were duped from the get go into using something that the founders knew wasn't sustainable and would eventually disappear. That's not a good way to treat users and it surely doesn't inspire confidence in the founders' next projects.
Honestly, I find pitching this service on the day of the news that Posterous is shutting down to be kind of tacky. I'm not trolling or trying to be negative...I'm just suspicious of why anyone should trust this.