That 91% marginal tax rate was only on paper. Most families (even high income families) paid 42% back then.
Also, no. It hasn’t been decreasing ever since. In 2014, the tax was 34%. It was raised higher, then lowered to 37% under Trump.
https://slate.com/business/2017/08/the-history-of-tax-rates-for-the-rich.html
If the income tax cap is higher for the same amount of income, (37 for 400000 moved to 39.6 moved to 400000) it means your income tax is higher as well. It’s called proportion.