What real estate would CN be giving up though? They would have the same number of tracks as they do today. Yes it may limit expandability on CN's part, but how many freight-only rail corridors in the GTA have more than 2 tracks? I can't think of any off the top of my head. The Milton Line through Toronto and eastern Mississauga has 3 for some stretches, but that may also be because of parallel GO service.
As for GO, the RLN is a Metrolinx project, so all that is under the same roof. If the RLN goes with this alignment, RER on the Richmond Hill corridor is redundant.
GO RER is needed north as well, just not yet. Metrolinx allowing a subway line along there is counter intuitive with the goals. Yes a Subway extension north should be done to RH. However, it should not be at a cost to the future GO RER.
The CN ROW is theirs to sell, lease or giveaway. Both CN and CP have a corporate need to future-proof their infrastructure.
The CP North Toronto subdivision is double track mainline.
Someone gets it.