Insofar as we need tax favored retirement vehicles, we have plenty—general taxation of capital income need not be concerned with that. There are two legitimate interests I see addressed by special consideration of long term capital gains (but the current “jist give it a lower rate” approach is suboptimal for addressing them.)

(1) Gains over >1 year taxed as current income in the year realized, when this is not repeatable (that is, when the owner doesn't have a large pool of long term investments to liquidate some of each year) is unfair in a progressive tax system because you will get high taxes in the realization year because of a high bracket, but you need to use the income across multiple years.

This can be simply addressed by allowing taxpayers to recognize income for tax purposes before realizing it (and/or allowing deferring some of the income received in a spike year.) I prefer combining both.

(2) Because of inflation, the real net gain from capital may be much lower than the nominal gains for assets held for a long time. (If appreciation is less than inflation, you may have a real loss with a nominal gains.) This is best dealt with by inflation-adjusting basis values when computing capital gains.