Expect major convulsions long before then. Once Europe begins to trade on one exchange, it would only take one other major player to announce a link to set in motion a rapid chain reaction. The result will be a single global exchange trading continuously, which raises the spectre of a rolling world market crash at some point in the not so distant future. Some exchanges might chose to remain outside the global trading alliance, but their longer term future will be uncertain. Both these barriers will be overcome, with time.
Confidence comes with use, and with confidence volumes and value of trades begin to grow.
Exchanges exist to serve their members and their members are banks and most banks are dinosaurs. This is a fundamental weakness of the structure of exchanges. From the investors point of view, trading on-line should be simple and low cost, enabling someone with a computer in a hotel room to directly buy and sell in the market. But members insist that all trades are done via members, who cream off profits for doing nothing except transmitting an electronic pulse from their server to the exchange. Members would rightly condemn the move because it would mean the exchange was setting itself up in direct competition with the banks themselves.