If you ride the Eurostar, there ARE intermediate stops possible -- Ashford & Ebbsfleet in UK, and Calais & Lille in France. Not all stops on any given train, and I do believe there are nonstop London-Paris services. But intermediate stations are served. Just like with the airlines -- sometimes you can get where you want to go nonstop, and sometimes you have to have intermediate stops, or even connect.

I agree with Gary -- some sort of regional services connecting with HSR makes the most sense for serving the most people at the most efficient speeds.