A properly designed ABI is capable of expansion. The design risk is not so much being backed into a corner, as just accumulating a great deal of obsolete cruft over the years/decades.
Win32 is a great example of this. It has been extensively overhauled, and best practice for writing a new application today is quite different from 25 years ago, but unmodified Windows 95 applications still usually run correctly.
The C++ standard library is a bit different from a platform API. The C++ standard library is a template system designed to conform exactly to your particular program. It should always be statically linked. I see zero advantage to dynamic linking of libc++. A platform API, on the other hand, is designed to stable, safe, and flexible, not fast --- because programs in general shouldn't be calling into the system so much. The performance costs we accept in a platform API (e.g., user to kernel privilege transition) would be totally unacceptable as part of an STL data structures implementation.
