I've seen C# interfaces and base classes that were committed years ago and never got a second implementation. This essentially makes the interface and base class a pointless ritual in hindsight.

It's always easy to spot in hindsight, but TBH I think the threshold for creating interfaces or base classes should be a bit higher - likely when you know you will need 3 or more implementations. It's always felt like premature optimization to me, unless something is hard coded to only accept an interface like using Type.IsInterface or something.

Concepts like "Product" are almost always going to have multiple implementations though, so it's easy to judge in that case to make an interface/base class at the beginning. The real magic is guessing what other concepts satisfy this requirement.