Every proposal I have seen kicks in after $100m. That's a level of wealth where even paying a 5% tax is likely to result in an annual net increase in wealth, as when you have that amount of money to invest achieving 5%+ returns is not unusual. The net result is that wealth would still increase, just at a slower rate.

Additionally, even amongst the general population, let alone startup founders, the number of people with wealth in excess of $100m is tiny. Numbers are hard to come by but I've seen estimates of 5,000 people in the US. What this means is that people arguing against a wealth tax are happy to disadvantage 330m people to protect the wealth of a low number of thousands.