I think the biggest risk in having an actual one-person billion dollar company is the Bus Factor [0]

Billion dollar valuation depends on investors, investors will never pay that valuation due the inherent risk in a one-person company. They’ll force them to hire for redundancy.

[0] https://en.m.wikipedia.org/wiki/Bus_factor

That assumes that the value is provided by the person’s ongoing skills rather than their skill/luck in coming up with the initial idea/brand/product.