Long term: yes, especially if you combine really high taxes in the 40+% range with consistently rubbish public services
But the real question is not whether people will leave, the question is how many talented hard working people chose not to move to your country in the first place because your taxes were too high. It won't show up in any data, you'll just experience worse economic growth and have to tax everyone else more
That would still be considered a fair and reasonable level of tax by european standards.
Obviously there is a level though - if you made it 95% then people would leave. I don't think there's any reasonable argument that the level doesn't exist, the question is where is it for a specific place at a specific time
You could say "well good riddance" but taxing $10mn of income at 20% raises a lot more useful money than taxing $0 of income at 95%
To my mind it's a revenue maximizing question. Is Massachusetts hitting that metric right? I have no clue. I do suspect that the voters and the people advocating taxes like this in Massachusetts and California are not thinking in those terms. I think they are doing this out of some sense that anyone making that much has to be doing something unfair and that an inefficiency in revenue is just money spent towards fixing that unfairness.
I'm happy to concede Massachusetts may very well have found the right balance, but there's plenty of studies on this in aggregate.
There's unsurprisingly a modest but statistically significant migration of millionaires from high-tax to low-tax jurisdictions.
https://www.asanet.org/wp-content/uploads/attach/journals/ju...