I'm sorry, but I'm not getting your argument. What does investing have to do with whether or not 37Signals' founders can walk away from their business?
I'm saying that with your definition of "walking away" (can stop giving a shit and the income still comes steady forever), not even people who do sell their company can walk away. All they're doing is shifting from a business that they love and have poured their hearts into (their startup) to one which they may well hate and which they may know nothing about (investment management).
Watch out for the inflation rate! If inflation is at 4% your ING account is worthless and you're spending down your principal in real terms.
Believe me, I've been thinking a lot about that recently.
That said, your point is taken: There is some amount of money where you can set up some fairly risk-free, static investments and take out $X every year indefinitely. Just make sure you do a little more thinking upfront about how much money that is, and what static investments those are.
TIPS or I-bonds would seem to be the prototypical example: they guarantee a certain real rate of return (about 2% when I looked a couple years ago, though I heard it's gone negative with the credit crunch), so if you've got $10M or so you're guaranteed your $200k. Only thing you have to worry about is a government default, but if they do that, you have bigger problems to worry about than what happened to your money.
