WRT inheritance, composition solves the diamond problem. It’s really about decoupling interfaces for portability more than code reuse.

I think the big abstraction problem inheritance solves is overrides. I think there’s a lot of FP cargo culting that overlooks the benefits of the ability to override (when used wisely). But aside from that I’m not sure there’s any reason to think inheritance is any more or less polymorphic than composition for a language with sufficient reflection/introspection support. If you lack type introspection it’s a different story. Maybe I’m missing your point though.