I bootstrapped a digital publishing business to over $200K per year.
Started 8 years ago, developed and published our first website for under $250 (I code, which helps). Learned SEO via trial & error, benefiting from the post-Penguin and Panda implosions that kicked a bunch of spam sites out of our target searches. Started putting together a predictive analytics package for investing in digital content, so we have an idea of how content will pull before we create it.
After that it was scaling and project management. Which has been a real learning experience, since a lot of publishing basically sucks... many people cranking out the same stuff. Very hard to keep "that special spark" in content at scale.
Still have the day job. Digital publishing is an 11 on the hot / crazy scale in terms of revenue stability (50% swings in monthly revenue on established sites aren't uncommon), so have a reliable pay check & health insurance reduces stress substantially. Plus I enjoy the work, most of the time.
One other benefit of having a "cover" day job: it allows you to "stay off the radar" as a small business owner and fairly affluent member of your local society. You've got a socially acceptable answer ("I punch a clock at company X") and don't stand out. Most people have no clue about the true scope and intensity of my side business, which simplifies things...