I do understand the requirement where checked baggage is involved, but there are many with carry-on where this isn't applicable.

Perhaps after passing through immigration, at YYZ for example, those with carry-on could use a separate exit directly from the second level that leads to domestic airside. Provided one doesn't continue down the escalators to the baggage claim area and has a domestic boarding pass in hand to show, plus hand in their arrival card. (Nexus only, maybe?) Anyone flagged by immigration would have to continue down the escalator.

There would certainly be savings to the airport in security costs and airlines would benefit from those whose arrivals are late and are facing tight/missed connections.

Although not perfect, I do like how YVR has a separate entrance to the domestic area from the International arrivals area albeit again with security.