I think the zero sum nature of HFT is already providing an answer. There is so much competition in the U.S. that HFT firms are having to expand to other market

I think the zero sum nature of HFT is already providing an answer. There is so much competition in the U.S. that HFT firms are having to expand to other markets like London and beyond that don't have the same level of saturation. There are only so many markets in the world, so in about 10 years (probably much less) when they are all saturated with very low margin HFT, equilibrium will be reached firms will simply evaporate and/or glom together.