The Ontario Line is obviously a necessity, but in the big picture, it's hard to think of someone not wanting density concentrated downtown to be any sort of problem.
Toronto wants so badly to be Manhattan, but we don't have the transit system required to support that level of density. We won't for decades even if everything that is planned dropped from the sky fully built right this second. And the focus on downtown-centric planning means that everyone with the misfortune of living anywhere in the GTA but there suffers; if there existed actual, real city centres in the suburbs (not pastiches of same like you see at Mississauga City Centre or Scarborough Town Centre), the quality of life would improve dramatically. Focusing all our economic and cultural centers in downtown Toronto is just bad planning, pure and simple.