Because that does not work with generic protocols. You can't have a function return a sequence:
func make_numbers() -> Sequence<Int> {
// Cannot specialize non-generic type 'Sequence'
}
func make_numbers() -> Sequence {
// Protocol 'Sequence' can only be used as a generic constraint because it has Self or associated type requirements
}
Whereas with this change you probably could at some point do something like this:
func make_numbers() -> some Sequence<.Element == Int>
