It will take a herculean effort to take control of the Milton Line. Basically building CP a new line for them to the York Subdivision. (and a new line IN the York sub, CN isnt just gonna share their tracks with CP) This is double digit billion dollar stuff.
In the time and money we could do this we could electrify the Lakeshore West, Kitchener lines with EMU trains, the Hurontario LRT to Brampton GO, build a Dundas LRT, finish the Mississauga Transitway and the 407 Transitway in the area.
While it would be nice to bring more service to Milton Line, you gotta look at what you can do instead with that money, which I think is a better bang for buck.