The hierarchical thing didn't really happen though -- there's no public SID registry. And machine/domain SIDs got pinned to 3 RIDs. So AD always had machine/domain SID conflict issues. It would only ever not have had SID conflict issues if they had had a public SID registry or if you had to install Windows as a domain member rather than install then join (and if there had never been a forest-of-forests feature).
Once you accept that machine/domain SID conflicts can happen, the value of having arbitrarily long SIDs goes away and you might as well use UUIDs to ID domains.
OK, perhaps hierarchical wasn't the correct word; it's not hierarchical in the sense of reflecting a (possibly global) domain hierarchy, but it does consist a component that identifies the issuing authority, and a component that identifies the principal relative to that authority.
So yes, a (UUID, RID) tuple would have worked just as well.
Once you accept that machine/domain SID conflicts can happen, the value of having arbitrarily long SIDs goes away and you might as well use UUIDs to ID domains.