Okay, I think I finally see where our inferential differences are and why we seem to be talking past each other. I'm retracting my previous comment in favor of this one, which I think explains my view much more clearly.

I interpreted the Repugnant Conclusion to mean that a world with a large population with lives barely worth living is the optimal world, given the various constraints placed on it. In other words, given a world with a set amount of resources, the optimal way to convert those resources to value is to create a huge population with lives barely worth living. I totally disagree with this.

You interpreted the Repugnant Conclusion to mean that a world with a huge population of lives barely worth living may be a better world, but not necessarily the optimal world. I may agree with this.

To use a metaphor imagine a 25 horsepower engine that works at 100% efficiency, generating 25 horsepower. Then imagine a 100 horsepower engine that works at 50% efficiency, generating 50 horsepower. The second engine is better at generating horsepower than the first one, but it is less optimal at generating horsepower, it does not generate it the best it possibly could.

So when you say:

All things equal a world with 50 Utils burning 2 million utils is better than one with 10 utils burning 10.

We can agree say (if you accept my pluralist theory) that the first world is better, but the second one is more optimal. The first world has generated more value, but the second has done a more efficient job of it.

So, if you accept my pluralist theory, we might also say that a population Z, consisting of a galaxy full of 3 quadrillion of people that uses there sources of the galaxy to give them lives barely worth living, would be better than A, a society consisting of planet full of ten billion people that uses the planet's resources to give its inhabitants very excellent lives. However, Z would be less morally optimal than A because A uses all the resources of the planet to give people excellent lives, while Z squanders its resources creating more people. We could then say that Y, a galaxy full of 1 quadrillion people with very excellent lives is both better than Z and more optimal than Z. We could also say that Y is better than A, and equally optimal as A. However, Y might be worse (but more optimal) than a galaxy with a septillion people living lives barely worth living. Similarly, we might say that A is both more optimal than, and better than B, a planet of 15 billion people living lives barely worth living.

The arguments I have made in the OP have been directed at the idea that a population full of lives barely worth living is the optimal population, the population that converts the resources it has into value most efficiently (assuming you accept my pluralist moral theory's definition of efficiency). You have been arguing that even if that population is the most efficient at generating value, there might be another population so much huger that it could generate more value, even if it is much less efficient at doing so. I do not see anything contradictory about those two statements. I think that I mistakenly thought you were arguing that such a society would also be more optimal.

And if that is all the Repugnant Conclusion is I fail to see what all the fuss is about. The reason it seemed so repugnant to me was that I thought it argued that a world full of people with lives barely worth living was the very best sort of world, and we should do everything we can to bring such a world about. However, you seem to imply that that isn't what it means at all. If the Mere Addition Paradox and the Repugnant Conclusion does not imply that we have a moral imperative to bring a vastly populated world about then all it is is a weird thought experiment with no bearing on how people should behave. A curiosity, nothing more.

Even if your argument is a more accurate interpretation of Parfit, I think that idea that a world full of people barely worth living is the optimal one is still a common enough idea that it merits a counterargument. And I think the reason the OP is so heavily upvoted is that many people held the same impression of Parfit that I did.