The Hamilton Authority has been quite outspoken in viewing Pickering as a needless competitor... to listen to the Hamilton folks, the whole Pickering project is an attempt from the big bad GTAA to waste government dollars to stop their plucky little airport from continuing to skim off some of Pearson's overflow, particularly its cargo market. One assumes the key factor in cargo movements is landing fees and relative placement of the airport with the regional warehousing and logistics infrastructure. Seeing as that's skewed to the western side of the GTA already, I honestly don't know if getting a new, eastern option is really super-helpful, but I guess that presupposes that acres upon acres of farmland along the 407 extension aren't going to be converted into Bramalea-style forklift sprawl by then.
Hamilton also figures in the question of whether we need another low-cost passenger hub. Thought it was interesting you mentioned WestJet... Although the Ryanair/Southwest business model loomed large in WestJet's initial planning in the mid-'90s, even in the pre-9/11 heyday of folks predicting that cut-rate airlines would conquer the universe and that Michael O'Leary was the smartest man on earth, WJ never really went wholeheartedly all the way to that end of the airline service model spectrum---their service has never been so self-congratulatorily austere as the real low-cost fundamentalists, and they've never had things like unassigned cattle-call seating. From my perspective, over the past 5-10 years WestJet's been sliding pretty systematically back along the service model spectrum towards being a regularly-priced orthodox carrier, albeit with some legacy trappings of its salad days (i.e. the one-class cabins, and the standard roster of flight attendant jokes about fastening seatbelts).
The reason I mention this is that when WestJet first moved into Ontario around 2000, the management still seemed relatively focused on being a Canadian take on Ryanair/Southwest, and the learned-wisdom from those carriers was to go into a low-cost peripheral airport, not the main internationals. So true to that form, they went with Hamilton, not Pearson, as their Ontario hub, and that was tried for about 3 years before that plan was discarded and they decided paying the higher landing fees for Pearson were worth it. Now, given the rate of change in the airline industry, nobody has the slightest clue what sort of service model WestJet will be running in 2027 or whether they'll even exist, but I think the WestJet Hamilton experience suggests that it's easier said than done to assume the carriers would be racing to move their flights away from Pearson and all the connecting flight options that come with it.
The other folks that have had some success with secondary airports are the international charter carriers, the Thomas Cooks and such, who don't care so much about being plugged into a feeder network of domestic flights... Gatwick is sort of a good example of an airport with that focus. Again, Hamilton's dabbled with trying to lure that sector (Flyglobespan was there briefly, which didn't quite work out) but it never quite stuck. Perhaps it's just a case of Hamilton being just a little too far away to lure GTA customers, and being unfortunately placed on the same side of Toronto as Pearson rather than having it's own lobe of 905 to itself. Pickering would have a leg up on them on both accounts there.