Quick thanks to everyone sharing their stories in this thread. I've been at this for a while, but only recently got to the point where quitting the full-time job to do the sites full-time might be feasible. This thread has confirmed I'm thinking about the right things to make that decision.
I don't see any difference between Google and the job market. Neither exists just so you can make a living. They both have other considerations, and you can run afoul of them despite your best efforts. And with both, it doesn't matter how good you/your sites are - if you're not sending the right quality signals, you won't get a job/rank well.
In the job market, some of those quality signals are at least partly under your control. Degrees, certifications, experience... but sometimes employers look for things as baffling as any algo - such as youth. Ageism is a huge problem in employment in the US. Companies believe younger workers will give them a better ROI, so they don't really even consider older ones.
Diversification is always the best way to ensure that even your lean times will be survivable. That, and as so many have mentioned, living well within your means and saving that money. Really, whether you work online, at a job, or both, the same rules apply. Bad things can happen, so try to create as many options for yourself as you can while things are good. Then the bad things only close SOME of those doors rather than the only one you had open.