Can't you work on anything you want now that you have the money? If you hadn't sold and some catastrophe struck you, what then? I'm sitting here counting out months of savings and wondering if I'll be able to build what I want to build before debt and daily life grinds me down.
He wasn't broke before the acquisition. A really successful product means you kinda get to do what you want whether you get an "exit" or not. Sure, the big cashout means you can go on a long vacation or something...but how many of us are in it for a long vacation? If I were to sell my company today, I would be thinking about what my next business would be by tomorrow.
A really successful product is not guaranteed to remain so, but cash remains cash. I speculate that if yahoo was going to get into social bookmarking then if he hadn't sold, they'd have picked up somebody else and the competitive landscape changes drastically. To me, the big cashout means an opportunity to work on bigger projects. The personal financials are of course the most important factor. If I had a few million or a fat trust fund, I agree selling only makes sense if I figure I am getting a very good price.