A reasonable actor is disincentivized when you give him less in return.
A stock trader is not so much an investor as a money manager. Disincentivized from what? Whatever he pockets as a stock gain the stock buyer eats... the net total system "investment" (a misnomer) is zero. His options, also, are limited. He either buys more equities or holds his money, and the latter is almost totally unproductive. We COULD, however, convert him into a REAL investor, by reducing his taxation rate on actual productive work.