I see your point. But tbh I was more seeing this as composition and with a parallel to protocol conformance more than a parallel to inheritance.
Also because one would just opt-in to a subset of cases than inherit a list of cases from a super enum, but also because we don't want the diamond problem
See it more maybe like one case would be similar to a protocol with only one static func (the enum constructor), and subenums being typealiases to composition of multiple of those protocols, if that makes more sense? (Even if I'm not sure the comparison is really relevant or the best one…)