You are right @Avi, you have described the classic sin.
But aren't we looking at another sin, then? Consider this other current discussion, which talks about removing indices. What will happen when the Standard Library defines the same extension, and a user imports Foundation? Or, more precisely, if Foundation defines the same extension (after all, IndexSet is Foundation's property)? Won't the compiler complain with ambiguity errors, due to two conflicting implementations?
Should SwiftUI be allowed to step on Foundation's "natural" territory?