"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."
I think you misunderstand this in two different ways.
First, retirees on fixed incomes are acutely, almost comically attuned to state income tax rates, property taxes, etc., and many of them plan their entire retirement and residency around it.
Second, although I agree with you that it is very difficult to just arbitrarily uproot and move your family, etc., because of a tax code change, it's not that difficult for wealthy people - especially those expecting a liquidity event - to buy a house on Lake Tahoe on the Nevada side and take the kids out of school for (365/2) days. "Remember that time we spent the ski season on the lake ?"
Wealth advisors and family office managers, etc., speak of this casually. Further, if you live in a wealthy community in the Bay Area (Ross ? Los Gatos ?) you'll see third or fourth cars with Nevada plates. Not an accident.
So, yes - I think you're right - nobody is going to stop living in California if we continue to increase the top end tax rate ... but define "living".