Seems to me that you need incredible amounts of money to be competitive in the frontier model arena. I don't know how much money Karpathy has to spend, but I'd imagine that the money needed would almost certainly mean investors with deep pockets.

And then there's the uncertainty, will the AI "wars" be some winner-takes-all situation? Will the smaller labs eventually be acquired by the bigger ones, will they simply wash away if there's a crash?

I don't know. If you can land some exceptional gig at the big firms, maybe the financials are good enough to not start your own lab. Minimizing risk, and all that.

EDIT: Assuming such a startup would focus on frontier models.