I had a coworker who did exactly this back in the '90s. He was an expert in a really obscure programming/database platform/language from the 1970s (called "Cyborg") that only had a few people left that knew anything about it. It took literally hours to compile even the tiniest code changes so his job mostly involved sitting around doing nothing waiting for the compiler to finish. He managed to eventually get a WFH situation (with dialup lol) that paid him $300 an hour, then went out and got two other similar WFH jobs that paid the same since his actual work load was just a few minutes per day for each. $900 an hour in the 1990s.