We (two python developers) have started a SaaS SEO checker service [1] in February 2017 (took 4 month to develop from 0) and already have paying customers on our business plan. I completely agree with the definition of successful business when you have ability to do what you want when you want. I already have a couple of other websites generating revenue from advertising and all this allowed me to quit daily job 2 years ago. So definitely there are a lot of examples of successful single- (two-) person businesses out there.