Your confidence is misplaced. On equality one might convincingly argue that by lowering cap gains rate we could increase the overall growth rate of the economy. By doing so we could increase other taxes on the rich without leaving them in a worse state than they would have been in the lower growth economy. We can then use this additional tax revenue to fund social services or otherwise redistribute income to reduce inequality.
On environmentalism, one might argue that we should cut cap gains taxes and, in order to make up the lost revenue, institute a carbon tax or other tax on environmentally damaging activities. This could have a profound positive impact on our environmental footprint.
Now, I'm not necessarily saying these are good ideas. But they are also not, prima facie, idiotic (to use your word). They are complex questions uncertain impacts. Reasonable people can disagree.
Your assertion about "macro 101 textbook and you forgot other people exist" is also quite strange when you consider the fact that I'm not talking about college freshman's stoner thoughts on optimal taxation of capital. I'm referring to the ideas of professional PhD economists who are well respected in their fields.