Percentages don’t scale well into the billions, you will still need brackets.
A billionaire can give away 98% of their wealth and still comfortably be a multi millionare.
A full time cashier on the minimum wage can barely even survive on 100% of their wage. When it comes to living a healthy fulfilling life, If they contribute just 5% of their wage to tax they are sacrificing far more a billionaire paying 98% tax would be.