There's a VIA train that goes from London to Toronto in 2h15. It departs London at 6:30am and costs $42. You arrive in Toronto at 8:45 - 30 minutes before the GO train. In other words if you want to go from London to Toronto for a commute, that's the train for you.
While I do agree that reverse direction commutes to London are a good idea, you seem to be missing the point of this extension. This is a pilot to gauge the demand of people wanting to commute to Waterloo from London. If this even gets a modicum of success, it can prompt the government of Ontario to invest money into upgrading the tracks which can see travel times drop to almost an hour to Kitchener. It is a baby steps first approach. On top of this, I'd argue that even the current proposition is fairly appealing to some. Many jobs in Kitchener start at 8am, and a 2 hour travel time is actually fairly competitive to driving during rush hours, and especially once Kitchener GO gets rebuilt, GO train users will be able to take the train directly to Kitchener where they can transfer onto the iON and get to work from there. Don't forget that Waterloo especially is a growing tech hub with companies like Google and Amazon opening major offices and branches there, and as housing prices increase throughout the GTA, the appeal of such a train especially once the tracks are upgraded would be quite high. But for now we just have baby steps.
Pearson? Idk about that. If someone's destination is Pearson, they're almost certainly more likely to use London Airport (which should probably be the first infill stop added to this extension later on alongside maybe New Hamburg) to fly to Pearson and connect directly to whatever flight they want, rather than sit on a train for 2+ hours first. What most likely the service pattern will be ultimately is all stops to Bramalea, and then express all the way to union with occasional intermediate stops, maybe Woodbine and Bloor.
