If we can eventually make some headway on the split base proposal, then eventually we might be able to let parts of base split off into separate packages that can update at different rates.
With that a change to the behavior of, say, something in STM, doesn't need to affect code that is written to a much smaller subset of the library, and so that people who are building Haskell compilers for non-posix-like backends can perhaps have a smaller set of things they'd have to implement out of the gate.
Alas, as you noted, that isn't the world we have to day.
-Edward