Let's put it this way. Simple common sense would dictate that if we increase the capital gains rate to 90%, very few are going to be intererested in making long term investments. So when you say there would be little difference between 15% and 24%, you are now making claim that YOU know what tax rate can be applied that won't reduce interest in long term investment. Because we already know that there IS a level that would shut investment off.
Actually economic theory would tell you that the higher the tax, the less of the activity. So activity gradually tapers down as the tax increases, until it ceases at some onerous level.
You know, when you make the statement regarding a 3X investment, what you evidently didn't consider is that some people might make such an investment, all other things being equal, despite higher capital gains rates, but then might well decide that because the rate is so high, that they would not sell at all.
Think this is not the case? Read about Warren Buffet sometime. His idea is to keep his investments....because the tax burden is high enough that it then discourages the idea of selling one to buy another....because each time you sell, you have a haircut to pay which means that your next investment must be that much more successful.
Now, lets say you make the 3X investment, but then you don't sell because the tax rate is high enough to discourage. This means that activity drops, and ultimately LESS money is available for long term investment. Remember, it doesn't take everyone acting in this manner, just some will reduce investment.
Now, if you don't think long term investment results in more jobs, and in general, higher economic growth, let me know. By the way, I know you understand that "long term investment" is not restricted to just stock market investments. It could be a private equity type of deal, venture capital, etc etc.
The bottom line is that increasing capital gains taxes will discourage some participants. For other participants, it will reduce their desire to sell at all, and therefore reduce their activity. There is little doubt that at the margin, activity will decrease, and this will negatively impact both jobs and economic growth.
OldTrader