Discussions on Passenger demand for Pickering routinely ignore how much catchments are shaped by transport options to the airport. In a world with GO RER, it's just as easy to access Pearson by transit from parts of Durham region, as it would be to drive to the Pickering Airport. I expect, any honest review would point this out.
And, for cargo to the area, it's probably better just to develop Peterborough a bit more. Justifying a new airport for cargo, over and above just investing marginally in Peterborough would be rather difficult. It's an hour's drive on the highway from Peterborough airport to where Pickering airport would be. Hardly onerous. Better than getting cargo from Pearson or Hamilton.
Pickering Airport proponents want to pitch this as necessary for regional aviation capacity. And as an avfan I get it. But as a taxpayer, I can't see the business case. Especially in the context of an existing airport an hour away (Peterborough), tens of billions in rail infrastructure being built in the region which will improve access to Pearson, and a possible $12-30B intercity rail line that would provide some level of demand relief to Pearson.