Example? Why not just use C++?

Because there are platforms with no, or really buggy C++ support.

That sounds like a someone else's problem. Find a platform that supports the language you need rather than try to make C into that language.

It is not C's job to be everything to everyone.

What is an example of an embedded systems project which truly benefits from a generic library in the way that you described?

Aviation would be my personal experience. We have to implement services on the aircraft for structured communication support with the ground tower. Those would really benefit from something like that, since we wouldn't have to implement a backend for every project and we could simply reuse the whole library everywhere.

I don’t see how generic programming (a la polymorphic types as presented) is the only or even the best solution to this problem.