Nobody is talking about "super airports" or shutting down Edmonton.
But economics is what it is. Better access favours the larger airport through economics of agglomeration. Just like how people today drive from Edmonton to Calgary or Ottawa to Montreal for better flight deals. If there's a train, those folks will find it even easier to get to the cheaper airport. And that cheaper airport will see average gauge of the aircraft servicing it, driving unit costs lower, compounding the advantage. Sure, people will still fly to Ottawa and Edmonton. But more and more people who live in Ottawa and Edmonton will take the train to get a long haul flight from the neighbouring city.