> Task/Task<T> does follow the pattern but they could have simply introduced a Unit type in the BCL and replaced the non-generic Task type with Task<Unit> instead.

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

Just because there is an obvious negative consequence to choosing a particular type schema (e.g. IEnumerable<t> extending IEnumerable) does not mean that the pattern is pointless. Sometimes biting the bullet is necessary since C# has a bullshit type system that makes dynamic orchestration a pain in the arse.