Fine. Now, when a business owner is thinking about the investment, what is the certainty of the rate of return? He may write something down, one number, but in fact there is a large variance as to what is expected. In addition, across investment opportunities.
Many expected returns are going to be so low that they won’t happen at all, taxes or no taxes. Others are so high that the impact of a small increase in the tax rate is going to be trivial. A few might be on the borderline, but that is always the case.
As lots of people have mentioned, our historically low tax rates mean that according to your theory there should be a boom in investment as more possibilities cross the RoI threshold. However, the poor demand climate means that expected returns are way down (taxes or no taxes) and that lowering taxes further would have a tiny impact.
If lowering taxes decreases demand by cutting jobs and benefits which get spent, it could make the situation even worse. Likewise, if increasing taxes increases demand, it could make far more opportunities profitable than are lost. Unless you have some data that there is a massive wave of investments just waiting to be made with a slightly higher rate of return, I’m not convinced that lowering taxes will help at all, and certainly not in a major way. Hell, usually returns have to beat the discount rate, and that is trivially easy these days. Why would lower taxes help when lower interest rates don’t?
If anyone was talking about doubling taxes, this might be valid. To turn it around, if the right is talking about lowering taxes more, doesn’t that also depress investment because businesses want to wait until they see if it happens? However, if someone is so chicken shit that they pass up a great opportunity in case taxes on big profits go up a slight amount, they have no business making such decisions. That source of uncertainty is tiny when compared to the other ones.