This would have massive impact on 'real' investors. If people could not profitably take on short term positions most of the liquidity in markets would dry up.

This would have massive impact on 'real' investors. If people could not profitably take on short term positions most of the liquidity in markets would dry up. Market Makers and HFT trading groups comprise the great majority of displayed liquidity (and this is precisely the liqudity accessed by 'real' investors when they wish to buy or sell). If that liquidity is removed spreads would widen and 'real' investors would take the hit. On a larger scale imagine a state pension fund or a large mutual fund that has $X billion which it needs to invest in the newly illiquid markets how much price impact will their 2M share order for Ford have now?