> Just keep the object files around and relink them against the updated version of the static libraries.

This wouldn't work, as extra care needs to be taken to not break the ABI of a shared library. We have various conventions to aid with this including but not limited to "so versions", but these must be kept in mind by the library developer. Object file ABI stability isn't even guaranteed by the compiler.

Also, these days with LTO, the linking step takes a tremenduous amount of time anyway. Doing this wouldn't help much.