First, any wealth tax being seriously discussed has a floor and/or has marginal rates, probably starting at 1 or 5 or 10 million (or higher).
Second, taxes don't disappear into nothingness - they pay for civilization. It is clearly beneficial to everyone to live in a society where people are well cared for and have healthcare, public education, welfare, etc. There's a reason failed states and unstable/developing countries generally aren't where people are looking to startup the next big tech company.
Third, any smart founder isn't going to just sell 1% of their stock every year and pay the wealth tax with that. They'll take dividends, or take out a loan against the value of the stock, or use some cash from other investments, or whatever, and maintain control of their company. Yes, over the long term they'll lose some wealth, but not necessarily control of their company, unless that's the decision they make.
Fourth, this effectively ignores that wealth is a thing that grows and compounds. If your wealth is increasing at 4% a year (very attainable for the class of people a wealth tax would affect) a 1% wealth tax really doesn't have as big an impact on your long term wealth as this makes it seem.
Fifth, the idea that people "will just move to another country" is very silly. If some people do leave, or start companies only in other jurisdictions, that just means there's a market opportunity for the many people who remain. Unless this supposes that no one wants to take advantage of one of the richest markets in the world because they might have to pay a small fraction of their wealth to the government. Not to mention that even very wealthy people likely want to live in a good society - we don't see many people starting companies on boats in international waters for a number of reasons (left to the reader).
I suspect that Mr. Graham is wringing his hands over potentially having to cut a large (in absolute terms, but small in relative ones) cheque to the government in the future, and I certainly feel for him, but I'd much rather we have well funded schools and welfare for those who need it.