> Microsoft could have added some unified Unit type but they did not, which makes this irrelevant.

It's not irrelevant - you're using Task/Task<T> as an example of the non-generic base type/generic subtype pattern being necessary, but it isn't.

IEnumerable<T> extending IEnumerable is a consequence of C#1 not supporting generics at all, and the non-generic version would not be necessary if it had. I was just using it as a common example of the boilerplate the pattern incurs.