If you accept the fact that 100% capital gains tax would result in no investment and a 0% capital gains tax would mean a lot of investment, surely there is a link between cap gains and someone's willingness to investment. Especially considering that capital holders tend to be sophisticated and highly sensitive to incentives.
So increasing the capital gains tax would lead to less investment.
So if someone prices out a project where they can expect 10% return but now you tell them they are only going to get 9%, then the risk may not be worth it.
You may not think like this, but I guarantee you the people in charge of allocating capital do think this way.
The point of the OP was that work is generally more valuable to society than capital gains, but is taxed higher so there is a mismatch between the worth and how we tax it. Saying if we tax capital gains 100% nobody invests, does not add anything to the discussion as the same argument applies to work.
Is it though? Capital build a better future. It's more forward thinking than just working.
For instance, I want my children to forgo working and build capital for their future by getting an education. The foregone income would pay back more in the long run.