The top 1% earners earned 21% of all U.S. income while paying 40% of all federal income taxes [0]. I would guess going from 50% to 70% would increase that share to would increase that percentage to 58%.
Old comment here, but I was assuming not just income tax to increase, but also to have capital gains, dividends, etc to be taxed at 70%. Current cap there is 20%, per my understanding, and a substantial amount of wealthy person's income is taxed at these favorable rates.
[0]: https://www.heritage.org/taxes/commentary/1-chart-how-much-t...