Well i believe LL has been toying with this for a few years now.
Mesh and texture content would obviously still be importable, although if this Brave New World doesn't accept sculpts then that'd make a lot of people pretty unhappy.
Closed source so we'd be stuck with their viewer and i'm trying hard to imagine how a locked down game could provide builders with greater creativity opportunities and freedom.
Anyhow it's still early days, and even if it actually does ever happen they won't be closing down SL; it'll be an alternative i reckon and they'll wait to see how many users they can rustle up before even thinking of this so i don't think we have to worry for a good few years yet.
Whatever, the resistance will be massive