I have no issue with a capital gains tax. Nor do most people. The bigger issue is with just taxing capital.

Also, this focus on capital is too narrow. It hides other forms of power that have captured our system: military, legal, media, etc... These forms of power obscure their power by redirecting attention to just capital.

Lastly, if you are concerned with external effects then tax the external effects directly instead of an indirect thing. So tax pollution rather than production in general.