IIRC the major issue was that we could't find an agreement what nested types would capture from the outer types (if the protocol was nested into another type, a protocol was nested into another protocol, a type was nested into a protocol, etc.) @karl should know better because it was his pitch back then.
In response to that and to solve some ambiguity I opened a thread in the early days of this forum but no one shared any interest or debunked the idea as complete garbage: