Are there companies (outside of Mozilla) that would hire (or at least pay) someone like that to keep working on Rust and its documentation? These people are too important to not pay enough. I know it's perceived to be the price you pay if you work for a foundation like Mozilla (or open source software), but it's not fair.

I've seen a guy who worked full time on open source have a Patreon, which had a few big sponsors that would pay / donate $5K or $10k / month - that's another good option IMO. Although I'd donate it directly to him, instead of having Patreon scoop off their share.