For what it's worth, I think this pitch as it stands is definitely valuable and is distinct from the implicit conversion issue. I've been guilty of misuing assumingMemoryBound myself when calling C APIs, so this makes sense as both a quality-of-life and a correctness fix.
As described in the other thread, I would find use in implicit conversions via value subtyping, but I think it would be harmful to Swift code to allow these specific conversions to be implicit when not calling into C.