I bet there are a few good businesses around building type hinting for large rails codebases, both for use for profiling, bug detection, and as an editor-agnostic backend. Ditto for large React codebases. I don't think these are large businesses, but look at eg Sidekiq. There's probably a very nice living for a handful of people who get to do interesting work.

NB: people on HN are oddly cheap. From the perspective of someone who makes payroll for 10 engineers, I can spend $1k on something like that basically because a senior engineer asked nicely or thought it might be useful. $5-$10k is definitely in scope for useful tooling. (Prices per year because I understand that if the authors can't make a working business, they stop offering me the X that I'm buying). Also, please make it work with vim. Pretty pretty please.

And there are probably good industry jobs eg at Dropbox for python or rust, etc. Basically, find a large company with a big investment in a slightly-off-the-beaten-path programming language, and there will be very interesting work.