There were a couple of interesting facts in an article by Coleman in the Tribune yesterday. I don't claim anything here, just stating them (from memory) for interest:
75% of the people on Shannon-Heathrow are going to England. These people may be served by other routes, e.g. Shannon-Dublin-Heathrow (revenge for the damned stop-over!), Cork-Heathrow, etc.
Shannon currently has flights to Frankfurt and Paris, two other highly connected airports, so the claim that it's going to be somehow cut off from the rest of the world is exaggerated.