"highly paid" is relative. $10k/month in San Francisco is an average programmer's salary .. Once you deal with the realities of a business (unemployment insurance, health insurance, payroll taxes, franchise taxes, income taxes, cell phones, internet, hosting) it's not really a single FTE's salary in say SF, Seattle, or NYC.
It simply isn't true that $120k/year is the average programmer's salary in SF. That's near the high end of programmer salaries, especially if you're young (programmers in their 20s rarely get >$90k).
There are plenty of programmers right out of college making low six figures. Of course, there are plenty of programmers in SF and SV; these are just the good ones.
If you want to live a programmer's lifestyle in San Francisco, then by all means keep your programming job.
For my part, I decided that not having to enter comments into a spreadsheet marked 'Game Changers' was well worth a 75% pay cut.
The 'realities of business' are not expensive for a sole proprietorship, if you are frugal. At $10/k a month, I could live like a king.
Assuming lead time is less than a year and the profits are already significant both of these options can grow extremely fast. However, just because you can pay someone a reasonable wage to do what you do does not mean it's profitable. It's easy to throw a lot of time and money after a bad idea that only seems to be working.