The original capital is not taxed, capital gains is.
Additionally, the higher the capital gains tax, the higher the expected returns have to be. Lower capital gains incentivizes riskier investment, which, within reason, is a net positive for society.
And I don't think we need to incentivize even more investment, given where things are at the moment. All it does is concentrate more market in the hands of the same people (who already have enough money to throw it around). It doesn't actually benefit the society as a whole, unless your metric for that is some mindlessly averaged metric like GDP.