You're right the markets will be fine but the overall strategies of investors are going to change.
As I said before, I'd selloff a major portion of my stock, reinvest and switch to long term holdings. I'm already seeing a few different strategies to avoid the tax.
The overall rate of selling is going to go down which leads to less revenue from capital gains tax.
https://www.wsj.com/articles/the-dumbest-tax-increase-11619384611
"As rates rise, Americans tend to hold on to their assets longer, reducing realizations. CBO has found that for each 1% increase in the capital-gains rate, there is a 1.2% reduction in realizations."
There's a couple articles that are similar on tone out there. I think I saw one video estimating like $100B loss in federal revenue over 5 years. Don't quote me on those numbers but I'll try to find it again.