I think the repugnant conclusion is exhaustively defeated by separating out what resources are used to build something with how those resources are used.

Given a fixed set of resources R, there are a variety of things W that you can do, and you can evaluate the effectiveness of doing so with an ethics system S. There's some W that is "best' according to S, and anything better cannot be accomplished with R.

If you add more resources to R, then you can do something like adding a single person, and you wind up better off. But W + 1 person isn't necessarily the best use of (R + stuff needed to add one person), as evaluated by S.

Of course, since it accepts the repugnant conclusion, there will be a barely-worth-living world Y' which it prefers to X'. But then it might prefer reallocating the resources of Y' to the happy world X'', and so on.

I'm pretty sure we're saying the same thing here, now. But I have something to add - preferring additional resources screens off the desire for Y above X. Once you separate world-building into magnitude (resources) and direction (distribution of resources), wanting Y above X means that you prefer larger magnitudes, rather than a different direction.