andreessen is talking his book, a16z benefits from more founders (more deal flow, more competition for their capital, more upside on portfolio dynamics), the "fate of the world" framing is investor PR
the data also doesn't support founder scarcity, the US has near-record startup formation, the bottleneck is durable success and capital efficiency rather than founder count, customer demand, distribution, regulatory clarity, and capital allocation are the actually scarce inputs
"give it a shot" is also -EV for most individuals, vast majority of founders lose money or earn less than they would have stayed employed, the survivorship-bias of "founders who changed the world" misses the 99% who didn't and are worse off for trying