To add to
@robmausser's point, the costs of tunneling (if the province were ultimately still hard headed enough to go through with that method of construction, had the Richview expressways lands stayed development free) could have been significantly reduced as cut and cover could have been used throughout the entire corridor. As a result, you wouldnt be using TBM asides from at geologically complex areas.
The fact of the matter is, Toronto screwed this project up from a cost perspective and the city has no one to blame but themselves. The province is likely saving the city from screwing it up further (operationally) by tunneling the entire thing. Initially I was against tunneling the line because the costs are exorbitant but since the city cant get its act together with enabling signal priority, this is one of the only was of getting reliable transit built. Obviously I would prefer elevated, but it would never fly in this area in a million years. The amount of NIMBYism here is enough to rival the amount we see in the Annex area.