Would you rather make $300k a year writing and selling proprietary software or fail/make $20k a year providing value-added services for an open-source project? This isn't a tough choice for me, but for some people the ideal of open source trumps all.
It's much easier to write software that fits a simple business model, than to figure out how to shoehorn a business model onto an open source project. You can tell which route is the pragmatic one: start from scratch, optimize for easy monetization.
That said, don't let any of this discourage you. It's absolutely possible to create a cool lifestyle business based on an open source project (or anything really). The only way to know if you can do this is to seriously commit to it. If you don't have to support a family it's probably a risk you can take.