Development expertise in enterprise tech is incredibly lucrative for freelancing/contractors. The day-rate for an on-shore AEM Developer gives me a small heart-attack.
AEM and Salesforce developers make a ton of money, especially if they are good. Contract Sitecore developers also make a ton of money, and never seem to be short of work.
The scariest money I've seen was from a guy that is/was an "ActionScript developer". He made most of his money travelling across the country to work on maintaining and extensing retail systems that used Flash as an interface, and ActionScript for backend logic. He was working for a client of my employer, and we got to chatting when we were in the city for a few days. I didn't get an overall figure, but he'd made £10k that week...
It didn't sound like he enjoyed the work, but most of his skill set was actively around legacy tech - ColdFusion, Cobol, Classic ASP, etc.
Unfortunately this usually means having built your prior career around said software as it's not always cheap /easy / possible to get a cop and self-learn. There are exceptions of course.