You’re right on about being an expert generalist.
But I think people focus too much on the second part of that term; I think we need to focus more on being an ‘expert’. That expertise will transfer over when one decides to venture into another track.
Your example of Elon Musk shows this already. He was great at computer programming, designing his own video game at just 12. After college, he started his own software company. Following that, he was CEO of Paypal. This was the amount of expertise he had before he began his other ventures. He was great already — he had much more experience than any of us would ever get in our lifetimes.