It is also my concern that the 9% top rate is just too high compared to comparable and neighboring states and could lead to a drop in net revenue.

A Tufts analysis projected that this would likely not be the case:

"Together, cross-border moves and tax avoidance would reduce millionaires tax revenue by roughly 35 percent [to $1.3 billion]. (Absent these responses, the tax would be expected to raise $2.1 billion in 2023.)"

Via: https://cspa.tufts.edu/node/406

However, there is substantial uncertainty here, especially in the long-run, that this end up going the scales for a lot of people, lower net tax revenue, and force cuts to state budgets.

Given that, I'm frankly mystified why the "no on 1" ads went with a "sympathy for multi-millionaires" angle instead of "risk of capital flight" angle.