Trading becoming centralized in the US is a different thing from trading happening exclusively on the NYSE.
As far as I understand, we are so far away from that reality since Reg NMS that we are facing the opposite problems, if anything: A proliferation of markets that brokers are obliged to trade at in the interest of providing "the best price", the combination of which has created a giant industry of latency arbitrageurs.
As far as I understand, we are so far away from that reality since Reg NMS that we are facing the opposite problems, if anything: A proliferation of markets that brokers are obliged to trade at in the interest of providing "the best price", the combination of which has created a giant industry of latency arbitrageurs.