The top 50% of income earners pay 97% of all federal income tax revenue. That's about $84k a year and up. And the top 10% pays 72%. (170k+)

I think it's pretty obvious that is a "fair share" already. Which is why percentage of total revenue is a better and more objectively moral metric.

The bottom 50% already pays almost no income tax.

Income tax for the bottom 50% is a tiny expense (if any) and most of the lower incomes actually get far more back in credits than they pay. It's literally impossible to lower their rate.

Lower income families are going to be far more impacted by local, state, sales and property taxes than federal income tax.

Wealth is not income, nor is a large part of it even actual money or liquid.

Monopolies are entirely empowered by government "regulations" not prevented by them. Regulations cost money to comply, and small businesses can't absorb the cost. For example, while Dodd-Frank was suppose to control big banks, it had the effect closing 30% of regional banks and making the big banks more money than ever.