North of Steeles along the general ROW, it is fairly dense commercial/industrial use, as well as crossings of 407, CN, Hwy, arterial roads, etc. I just don't see it as any kind of open ground. Granted, some of this infill could have been prevented by land acquisition back during planning phases.
Although it is for 'the public good', expropriation is a fairly heavy handed approach to land use that, if used on a large scale, might send a chill to industrial/commercial investment. You could use the same argument for the Scarborough extension or DRL, it becomes a matter of how many lives and businesses you are willing to disrupt.
Given that it has to be done by land parcel, I could see it getting it getting convoluted if done on a large scale. As we have recently seen, governments using heavy handed tactics to achieve its ends isn't always popular.