Reducing duplication in third-party libraries should not be used as the slam-dunk rationale for this proposal while all those same libraries also define a dummy protocol for Result to conform to to allow it to be used in where clauses and so on. Making Error self-conform would erase a major one of those limitations, but it's not the only limitation.
Those obviously aren't and shouldn't be in the scope of this proposal, but it's not valid to say "we should merge this in posthaste because it'll unify the community", because it won't.