I think Kevin J. Connolly is on the right track below. Although I have important disagreements, his observations seem to be grounded in serious thought about how the registry-registrar relationship would actually work operationally and contractually, and I respect that kind of contribution. If one believes that that should be done, that's for governments to do. I think that view commands a consensus among the SOs, and especially among IETF-ers who think seriously about the issue.
Auctions are another method.
KJC's proposal of a public benefit calculus is worth considering, too. But the real answer is that this is, fortunately, a problem we do not need to solve right now. Remember, we are just dealing with a testbed of 6-10 registries. It can also base its choices upon the principle of avoiding avoidable short-term problems. Long term, of course, it will have to deal with that issue more systematically.