I know of a guy, but his scenario was quite unique. I was working for an energy company who shall remain nameless, but who's internal IT was a tangle of external consultants milking the place for millions, and ineffective/underserved full time staff who couldn't run a query on a database without a change control committee of consultants milking them for yet more cash.
Anyway, this guy was the go to guy for gas customers, and knew the database inside and out. So he created his own company, resigned as a full time employee, waited until the panic had set in properly.
Then he offered consulting services back to the energy company saying he'd take care of any database processing costs, or cloud migration costs or whatever, and moved the customer data for gas customers to his own system. Then he created an API, waited a while more and said he was going away again.... Or he could stay supporting this setup if the energy company agreed to a monthly fee and API usage. Then, as far as I know, he sat back and just watched the money roll in while he automated everything else about the job.