I picked examples that come up commonly in the field, but I don't think it's unreasonable to also want to extend a framework class to conform to a protocol you control. (If you look at the previous thread I linked, there are further examples from users trying to do exactly that.) The questions around retroactive conformance to protocols you don't own are separable, and the fact that Cocoa has run into these issues seems to me like a good indicator that other developers' libraries would as well given enough time, even if it's too late for us to do anything about Cocoa itself. (Making a required initializer like UIView.init(coder:) into a required!-with-trap one would be a non-source-breaking change, so it isn't out of the question that we could modify the compiler to take advantage of something like this given enough time.)