It's a simple idea (figuring out what makes you happy is the actual hard part). If you're happy, then you're already thriving; it doesn't make sense to say "I'm happy, but I'm not thriving." What's the point of thriving, after all, if not life satisfaction?
I think your friends have a narrow-minded view of what makes life worth living. I'm not saying they're wrong about their chosen path; they're just generalizing from their experience to everyone else, and that seems foolish. Everyone has to find their own happiness. For some, it's building a company. For others, it's learning, writing, etc. Neither way is objectively "better" than the other.
There was a great Onion article recently that parodied the fallacy of assuming others are failing if they don't share your goals:
http://www.theonion.com/articles/unambitious-loser-with-happ...