"I didn't see it then, but it turned out that getting fired from Apple was the best thing that could have ever happened to me. The heaviness of being successful was replaced by the lightness of being a beginner again, less sure about everything. It freed me to enter one of the most creative periods of my life."
Now, Steve didn't flip Apple, he didn't sell out. But the point is that he did start over from scratch. 37signals are right that building a business for the principal purpose of flipping it is probably a perverse goal. But being a serial entrepreneur is not a bad thing, and Jobs is precisely an example of why it's not a bad thing.
I'd also add that Warren Buffett isn't an entrepruer in the same sense that someone running a tech startup is.
Exactly, it's rather silly to expect a tech entrepreneur to follow the same career path as Warren Buffett.