This assumes that the primary reason to start a company is to become fantastically wealthy, which I would contest.

Sure, most founders want to get rich, but I'd assert most would be just as happy to end up in the 8 figure range ($10's of millions) as they would in the 9-10 figure range. (And such a reduction would require taxes on a scale far beyond what UBI would require)

Few people are going to consider starting a company and then sit down, calculate their tax burden as a billionaire and decide that it isn't worth starting Apple, Facebook or Google.