But then what?
GWT is still around and developed, but they've moved on to the next big thing, i.e., dart.
280 North, the Cappuccino developers, was bought up by Motorola a year ago for $20 million.
Strobe, the primary developers of SproutCore, were bought up by Facebook a couple weeks ago.
Since these projects are all under Free (MIT, Apache, and LGPL) licenses, they won't disappear (in fact, they're still receiving minor updates). But when their primary (or only) developers get bored or cash out, it's safe to say they'll soon stagnate, which is a shame, really.