- Build, not exactly -- only in the sense that they can handle minor upgrades via bitcode for new ARM variants, if you choose to enable that.
- Moderate/curate, sure.
- Provide a lucrative channel? Yes, but only because they've made it the only channel by fiat.
I'm not sure I see 30% worth of mandatory value here. Is Apple's moderation so good it's worth 27% the cost of my app? Do developers think they're paying for bitcode upgrades, and do they think that justifies the fee? Recompiling for new minor architectures isn't difficult.
For that matter, is there more value provided in moderation for a more expensive app? If they approve my app at $5, and then I up the price to $25, did their moderation process retroactively become more valuable to anyone?