As much as I hate giving vehicles priority, Yonge Street is a major artery downtown.
If you close it, what options are there for alternatives? Are you going to send everyone down Church Street or Bay (which is already a major thoroughfare itself)?
At most times of day, Yonge north of Queen is relatively empty. I frequently cross mid-block without needing traffic lights, and without needing to sprint across, I can just stroll. There is ample surplus capacity.
Less so at peak times, but this is not a case of displacing large number of cars, at most times.
Also, a 1-lane (per direction road, where those lanes are never closed for construction (of buildings), and where there is no lane-shifting by cars, will actually be more efficient than the current set up and hold higher volumes of traffic per lane km.
At peak times, there isn't much surplus capacity on Bay, but there is on much of Church, so there is some diversion capacity available.
Right but the vehicles are not just going to remain parked in the driveway because someone closed a street. They need to go somwhere.
In addition to what I've noted above, its important to say that in fact, reducing capacity for cars does result in at least some marginal reduction in demand. (cars that stay parked in drive ways, to use your parlance).
When you make it more attractive and pleasant to choose walking, cycling or transit, more people will make those choices, when you make it less convenient to choose a car, people will choose it less often.
In combination, its not difficult to imagine an absolute reduction in demand of 25% or more.