The reason why you won't convince people with this argument is that for you, it always comes back to gridlock. For you, gridlock is the primary problem, and I suspect this is because you are a commuter. For people who live in Toronto, gridlock is not the primary problem. Livable streets that aren't just commuter highways is what many people in Toronto want. Bicyclists also want to be able to drive on streets without being hit by cars.
As well, the 401 - one of the largest expressways in North America - is always packed with cars, so even if gridlock is the primary problem, it's clear the current number of commuters using cars is too many. If we want to avoid gridlock, we can't add more streets, so we have to get rid of some of the cars. I think you argued that this could be done through increased transit. This shouldn't be a zero sum game: we should increase transit, increase alternate forms of transportation (such as bicycles) and give drivers disincentive to use their cars - all at the same time. "Making things worse" by reducing the number of lanes for cars could, actually, make things better.