You may be right. A small number of startups may be founded elsewhere. But the large income from a wealth tax likely provides a greater benefit.
We've had a wealth tax in the past. It worked. The only reason why it got removed was because the super rich used their vast influence. It's not something that's in the public's best interest.
You do realize that most European countries that has a wealth tax abandoned it and the recent ones never actually collected anything close to what was forecasted because it’s not that hard to move wealth around.
Sure, not every founder will make that choice and not right away, but you could see a slow shift over decades to more tax friendly locations.