The purpose of capital gains relief is to spur investment and real wage growth. Unfortunately, the JTC and Treasury maintain they don't know enough to estimate the additional economic growth that would result from reducing capital gains taxes. So they assume no additional economic activitiy. One consequence is that, as we have learned repeatedly this year, when the economy grows faster,Treasury revenues grow faster. More jobs mean more personal income tax.
Higher wages mean higher tax brackets.
More profits mean more corporate income tax revenue. In fact, virtually every tax imposed yields more revenue when the economy does better. Thus, a capital gains tax cut would increase the revenue flow to the Treasury over and above that due to an acceleration of capital gains realizations. For tax distribution purposes, though, it's important to know who will pay these additional taxes. The overall federal tax system is highly progressive.