> If you're not used to it, it looks like you've over-engineered and ended up with Generics soup

Yes, it looks exactly like that. You’ve built not just an interface for UserStore you probably didn’t need, but a generalized Store abstraction that you definitely didn’t need.

> but paradoxically, you're committing yourself the least to any concrete implementation

This isn’t paradoxical at all. You’ve avoided committing yourself to a concrete implementation by implementing layers of goop you don’t need and likely never will. And because you did all this abstraction without real needs to ground it, you almost certainly did it wrong even if you do eventually need it.

You are gonna need a user store, so you should have built that first.