[self_awareness]

I think you've navigated away from the scope of my remark; I've specifically asked what's the point of using interfaces when there's just one implementation, not "what is the point of interfaces" in general.

[deepsun]

I think parent means that the one implementation may be changed in the future, so everything applies.

If you 100% sure the one implementation will never change, then I'd say you're right. But it requires future-telling.