So what do you say to those who claim to be hopeful universalists but reject the possibility of UR?
Not sure I follow your analogy - if hell exists then it's patently unreasonable to expect nobody would be in there. But to hope it could be emptied by the one who holds the keys, and who comes to free the prisoners...well only a very poor judge would deny that's eminently reasonable.
I don't quite see what you're saying. If it could be emptied then once that happened no one would be there. Furthermore, it's almost certainly not a physical place with a literal key. Saying that it might exist with no one in it doesn't mean that somewhere in the basement there's an empty room. It means that it's a possible condition that no one is actually in. It seems like hopeful universalism says it is possible and maybe even probable that universal reconciliation will happen.
Traditional theology has often understood universalism as saying that there is no accountability, because the standards are so low that everyone gets in. Hopeful universalism seems to me to say that there are in fact real standards, so failure is conceptually possible, but God may enable everyone to meet them. It's possible that no one has actually held universalism in the first sense, and that actual universalists are hopeful universalists whose hope is a bit stronger.
What I mean by saying that hell exists even if it's empty is that there are standards, so that failure is conceptually possible, although we may differ on the likelihood of people actually failing. (Language about standards needs to be understood in the usual Christian context where it's not just an accounting of good and bad deeds, but Christ intercedes for us, and our faith matters.)
I avoid taking a dogmatic position because I don't think we have a clear picture of what will happen. Scripturally I think the most likely picture is reconciliation of many (I hope almost all), but destruction of some. But in saying that I'm looking at hints in the Gospels, and I'm combining 1 Cor 15 (which itself has some ambiguity) with hints in the rest of Paul. So I'm far from confident.
Despite attempts to construct one, there is in the Bible no actual description of judgement that answers all the questions we want answered. The Revelation comes the closest, but it's sufficiently symbolic that it is open to multiple interpretations, although probably not UR. To be honest, though, I don't place much weight on the Revelation.
Further complicating things, it's unclear what Jesus could have expected his audience to understand by the terms and images he used. He spoke of destruction using OT images and traditional Jewish terms. But Jews interpreted these in different ways, and most of our evidence comes from later. Gehenna, for example, is not an OT word. It was commonly understood as eternal, but anywhere from many to all people eventually got out of it. There's also some indication in the Talmud that it eventually ends, and / or people who didn't get out were eventually destroyed, but it's not clear how common those ideas were. The OT language if taken in the OT context suggests destruction, but that language was used by Jews to refer to Gehenna, which is potentially eternal punishment.
Because he didn't say anything more explicit, and (particularly in Luke) used such a variety of different ways of speaking, I would understand Jesus as talking about accountability, but not the specific form it would take.
