I YAGNI the concretion and write the abstract-as-possible version.
Do I write a UserStore? That would be the simplest, right? Well no, I might not need that particular formulation of a User. So I just make a Store of anything-which-is-storable.
If you're not used to it, it looks like you've over-engineered and ended up with Generics soup, but paradoxically, you're committing yourself the least to any concrete implementation.
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.
Personally I prefer SOLID to YAGNI.