Right off, the study's conclusions do not match the assumption that "income inequality doesn't matter."
The conclusion suggests more studies on the relationship of taxation to total lifetime spending inequality over the lifetime of those in the various income quintiles. The takeaway from this study is "Inequality and fiscal progressivity shouldn’t be studied in isolation or in a piecemeal fashion, nor can they be accurately assessed by mixing very different age cohorts in the analysis." That does not mean that income inequality does not matter, it means not taking one aspect of the study of income inequality and amplifying that meaning over all others (p. 34 - 35.)
The study also has to make a series of assumptions on everything from tax incidence and results on capital, to labor force participation factors, to mortality based on not just income quintile as a single "cohort" but also system dependencies such as Social Security, SNAP (the study calls Food Stamps in the traditional sense,) Medicare and Medicaid, etc.
Economic studies are known for assumptions, and while the vote is out on these assumptions being spot on or not the data suggests the conclusion as stated. Not some asinine assumption that income inequality does not matter. Further, the study does not suggest the removal of all government methods to try to help income inequality (as the "Solution Method" to the math, p 47, indicates on applied impact over the lifetime of those at various income quintiles.)
"What does matter" is what we have always known it to be, doing just enough to ensure healthy participation in our economic model across all income quintiles balanced against not doing so much as to upset economic conditions for growth and healthy return. It is not all or nothing, never has been. The study itself makes some conservative lean assumptions on the relationships of taxation to economic impact but in itself does not suggest some utopian trickle down economic theory either. Again, balance.
Included is the actual study (that I doubt many of you would read anyway)...
http://www.kotlikoff.net/sites/defa...gressivity, and Marginal Taxation 3-14-16.pdf
And I forgot to add... right in the "study"... P1
"NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications."