Don't forget that you don't have to cash out of a successful business to move on to other things. You can install managers to grow a business once you figured it out and become an owner rather than a doer.
I see myself doing this eventually.
So when you say something resembling "you'd expect to see companies like 37signals trying to roll the snowball into something like Twitter but that rarely ever seems to happen", I'm inclined to point out how that misreads the dynamics of companies like 37signals.
† (won't bore you with details, but I have many of them)
Fried and DHH make enough money from 37signals. They are shooting for the moon by contributing to open source and writing bestselling business books.
Because you might have fallen in love with the idea of creating the next big thing? The thrill, the excitement, the challenge of taking a shot at the next big thing?
Joel Spolsky is a great example.