There is a third option, which was brought up by Chris Lattner and discussed in this thread. (As per the grand tradition of these forums, the thread is about a completely different topic, and the relevant discussion is intermingled haphazardly.)
Essentially, instead of making the whole standard library ABI stable all at once, he suggested taking an incremental approach. There could have been both an ABI-unstable “baked into each app” standard library like there used to be, and also a new ABI-stable dynamically-linked standard library distributed with the OS like we have now.
At first everything would live in the unstable library as it had been. Then, slowly, over the course of time, when each individual piece of the standard library became fully optimized and finalized, it could be moved into the stable library.