I think about this a lot - I made the jump from salaried worker to self-employed programmer/freelancer/contractor/consultant (depending on how you look at it). But I'm doing enterprise-level architectural programming. It's not very amenable to productizing, so I'm basically stuck being paid for my time.
What that means is that if I want to have a long-term high-security relationship with a couple of key clients, I basically hit a ceiling in hourly rate (a good rate, but slightly below what your general multi-person agency will charge) and I've been at that ceiling for a couple of years now - nowhere close to $250k .
It appears the way to transition out is to bump the rate up significantly so it doesn't appear you're trying to compete with agencies - maybe around $200/hour or 6k-8k/week. But the problem with that is that the long-term relationships can go out the window. They only want to bring you in for shorter periods of time, and you have to hustle more. So the marginal benefit isn't awesome. And I really like my clients and I hate hustling, so I stick with where I'm at. This higher rate I've had for the last 2-3 years is nice and all, but practically what it means is that I'm projected to be able to retire at age 65 instead of age 75. It's not a dramatic difference in lifestyle.
So instead, I've just been trying to amass a larger emergency fund outside of maxing my retirement savings every year. And then I'll do something with it... what, I don't know.
I know, first-world problems, but it seems that now that I finally have a bit of a nest egg, the income-maximizing move would be to use it to invest in one of these other careers the forbes article mentions. But what? Nest egg money isn't really the limiting factor behind a retail operation, it's the actual retail product idea, and I don't really think that way. What's left is real estate, but I'm regularly told I shouldn't even bother if I'm not a fix-it handyman sort, which I'm not. So I'm sort of in this state of perpetual mulling. I wonder if I could just find a talented-but-poor subcontractor, get my own general contractor license, and handle all the business and project management parts (which I love) despite knowing nothing about construction.