If we spent a few hundred million to get the Toronto-Montreal and Toronto-Ottawa trains down to a reliable 3.5 hours that run more often than 5 times a day, you could eliminate the need for airport expansion for decades. If you actually built a real high-speed train, you would never have to expand an airport again. Of course, it makes much more sense to spend billions of dollars, pave thousands of acres of land, and massively increase our carbon emissions to accommodate everyone by air. It's fair competition.
The point about it being ridiculous that the site can only handle 50 million people is totally right. Atlanta, Chicago, Heathrow, Frankfurt, and LA are all in roughly the same size class as Pearson, and they all handle far more people.