A startup can be a healthy business with revenue -- of course -- but 37 Signals is not a startup. 37 Signals is a business in the traditional sense; they're focused on generating revenue instead of an exit.
From wikipedia: "Lifestyle Businesses are businesses that are set up and run by their founders primarily with the aim of sustaining a particular level of income and no more."
And that's great if you want to create a business that sustains your lifestyle.