That's an indicator of wealth inequality, no? The top 50% are paying less as a % of their income, but because their income is rising faster (relative to the bottom 50%) than their total tax rate is falling, they end up paying more in total, mainly because the bottom 50% have less to give.

Is that the case if you include payroll taxes? (We could include sales tax too, but those are usually state and local, so we'd then have to add state income tax, and possibly property tax, and that would get complicated)