I think I have just as good a shot at building what comes after git as their team does, and perhaps quite a lot better.

I'm not famous though, I'm just a good engineer who is patient, inquisitive, and determined enough to spend the last five years of my life on nothing but this.

My question is: say the investor believes that some new platform will win out over Github. How do I make the case that it will be mine over a famous person's?

No offense, but why should I believe you? The guy is famous because he has a track record of success doing similar projects. Of course that doesn’t guarantee success, but I’d wager it makes it statistically more likely than a random person. Starting a successful company is not all about good engineering.

Have you built a prototype and tried to pitch any VCs? Or are you just asking rhetorical questions?

Sadly you can't, and this is a huge flaw in the venture capital model. They invest in people that they think other vcs will invest in. More often than not who your parents are matters more than anything (unless you've had a huge exit like the OP). They'll also throw money at you if you come from a rich family, not because they think you'll succeed, but because they want your family's money as LPs in their funds.