The current arrangement of having different concourse areas for intercity rail (VIA), regional rail (GO) and the Airport link (UP) is already questionable enough and considerably hampering the ease of transfers between these different rail types. Moving the VIA concourse two (!) levels up would wildly exacerbate this issue.
The problem is that cities like Hamilton, Kitchener, London and Windsor are too small to justify their own pre-clearance facility, but too big to skip these stations. You will never fill an entire train with international travellers when starting in Toronto amd you will never convince American border staff to man railway stations in cities like London, Kitchener, Hamilton or Oakville. This is fundamentally different for London-Paris, but the Eurostar stations Ebbsfleet and Ashford have not been closed by choice, but by necessity (a severe lack of French border personel to staff these stations).
Good for you, but this is not how hard borders work.
The border facilities at London Saint-Pancras are extensive (and I’m certain that that picture only shows a small fraction of the facilities at Gare du Nord - even Lille-Europe is far more extensive!) and still so insufficient that Euostar has to artificially restrict its passenger counts by 30% to not overwhelm the border facilities: