Sitting on the north-eastern fringe of the GGHA, Pickering seems like the wrong location for a new international airport. It should be in a more central position so as to benefit more people who live within a 1 hour drive radius. From my limited perspective it makes a ton more sense to massively expand Pearson (two more piers + huge transit terminal/processing center + new runway) over the next two decades and consider additions to an existing airport like Hamilton. There are plenty of airports in the world with smaller footprints than Pearson that handle much more traffic.
Additionally, the economic benefit argument is a non-starter since it equally applies to any large new chunk of infrastructure--whether it be HSR, a new subway line or an airport expansion. The only difference is who pockets the benefits from said infrastructure project. (Which is why corrupt, authoritarian states LOVE giant new airports while free market democracies rarely attempt them any more--seriously, are there any new international airports being built in N America?) Anyway, I digress. As a non expert on these matters I'll leave it at that, and wait for stronger arguments to alter my current position.
On edit: sorry michael_can, didn't mean to parrot what you said; just saw your post after I posted mine. =/