Although I completely understand the "every man for himself" opinion on the passengers' side, I still think (until they fix the flaw) that UA, when alerted to R availability and existence of waitlist, should do the right thing and clear based on waitlist.
There's two responsible sides to this:
1) Passengers - every man for himself until UA fixes the problem
2) UA - fix the problem and, until it's fixed, clear the waitlist in order when alerted to R availability and waitlist.
And Channa, that would be a crackling version of Rhapsody in Blue