We should wait and see if "rage unaccepting" becomes a systemic issue.
And we should be very careful before we conclude something constitutes "rage unaccepting."
People who are unsatisfied with existing answers should unaccept them.
One user appears to be unaccepting vindictively, and we're considering a policy change?
This doesn't indicate a trend and it's silly to think something must be done about this, unless there is more information to indicate this is becoming common.
Furthermore, of those three posts (and the unaccepted meta post), it's not clear that any of them are really rage unaccepts.
I don't see any strong indication in any of those questions that the OP was actually satisfied by the answers. It seems perfectly plausible--and, by Occam's razor, pretty likely--that s/he had felt pushed into accepting answers. Now that they don't feel pushed into accepting answers dishonestly, they may simply be doing the thing that is both good for them and honorable toward the community: unaccepting answers they should never have accepted in the first place.
I have seen this happen on Ask Ubuntu, where users admit to accepting answers (often total crap answers) just to make their accept rate increase. As the social flogging for low or medium accept rates let up, they unaccepted answers they felt they'd been bullied into accepting originally. (And the social pressure to accept answers on Ask Ubuntu was considerably lighter than it sometimes was on Stack Overflow.) Unaccepting those answers was the right thing for them to do.
Reputation we earn from wrongful accepts is dishonest reputation. It doesn't reflect badly on us (because it is not usually our fault). But it harms us. Like serial upvoting, it dilutes the real-world meaning of reputation.
From time to time, someone accepts an answer of mine when it's quite clear it didn't solve their problem. For example, they might accept but comment about how it didn't work. When this happens, I explain that they should unaccept the answer. This is not a selfless act; it is not motivated just by my concern for them. It is both practically and morally bad for me to have reputation that comes from baseless acceptances.
Losing reputation from such unacceptances is like the sting of cool water on sunburned skin. The injury was already there, and as it starts to be fixed, we become aware of it.
If people really are "rage unaccepting," then that is (at least a little) bad and harmful.
But what we have is one possible rage unaccepter.
Now, let's suppose that the answers this user unaccepted helped the user and should have remained accepted.
Does that mean the user is malicious? Or that s/he has just judged poorly? Or that s/he doesn't understand how accepting answers is supposed to work because s/he never learned to accept answers properly?