No empathy here. Welcome to living in the centre of the city. What do people expect along a commercial/employment throughfare designated as a major collector in a rapidly growing city? Even City staff acknowledged that nearby intersections would still operate with residual capacity - there are impacts, but the effect on Overlea has been exaggerated. Given the slow deterioration of the East York Town Centre and the surrounding employment lands over the years, it's hardly as if the uses on or near Overlea have been generating additional traffic over the years. Additional traffic volumes have largely been the result of through traffic, and that traffic will find alternatives if Overlea becomes more congested. While I sympathize with those stuck with bad transportation planning decisions made in the 1950s, but it's even worse planning to downzone/sterilize lands on a major collector nearby because the municipality has failed to invest in adequate transit - we'd simply be adding more bad decisions to the pile of bad decisions.
I would also note that the primary voice on the traffic issues has been, as far as I can tell, the Leaside Property Owners’ Association, who likely have little concern about traffic to and from Thorncliffe Park, but are far more concerned that some of these Costco shoppers might end up taking Millwood to Bayview, or God forbid, McRae Drive.
It's 2017. A retail/warehouse use like Costco isn't going away any time soon. This is precisely the type of location where this type of use should be located - on a major collector road surrounded by employment and commercial uses.