> I don't think it's unreasonable to have a system where every program uses the same version of a library.

I really think it is. Even at the scale of a single app it may sometimes make sense to have multiple versions of a same library, if for instance it implements a given algorithm in two different ways and both ways have useful properties

I have seen this (linking with multiple versions of the same library) for maintaining backwards compatibility, for example to support parsing a data file from a previous version, but never for selecting different algorithms.

Then shouldn't these APIs be exposed as different libraries?