I’ve always thought the Repugnant Conclusion was mostly status quo bias, anyway, combined with the difficulty of imagining what such a future would actually be like.
I think the Utility Monster is a similar issue. Maybe it would be possible to create something with a much richer experience set than humans, which should be valued more highly. But any such being would actually be pretty awesome, so we shouldn’t resent giving it a greater share of resources.
Humans seem like (plausible) utility monsters compared to ants, and many religious people have a conception of god that would make Him a utility monster (“maybe you don’t like prayer and following all these rules, but you can’t even conceive of the - ‘joy’ doesn’t even do it justice—how much grander it is to god if we follow these rules than even the best experiences in our whole lives!”). Anti-utility monster sentiments seem to largely be coming from a place where someone imagines a human that’s pretty happy by human standards, and thinks the words “orders of magnitude happier than what any human feels”, and then they notice their intuition doesn’t track the words “orders of magnitude”.