This only matters for synchronous functions though. And I don't want to trivialize that, it is an important and common thing! But, a non-isolated async function + a Task will behave identically.
Oh, and while you cannot do this today, eventually I hope that this proposal gets accepted. And, then we'll be able to write this to explicitly remove isolation at the definition site of a closure.
Task { nonisolated in
print("nonisolated")
}