I concur. Using the 2006 Census, there's over 265,000 people in northwest Mississauga (north of the 403, west of Hurontario). That's a big chunk of population that shouldn't be ignored; about 40% of Mississauga's population. In fact, that one quarter of Mississauga is home to more people than the entire city of Vaughan. By comparison, only 11% of Mississauga's population lives south of the QEW. Milton could add at least 100,000 more people to that corridor.
With fare integration and frequent trains, such a re-routed line could easily become a "Mississauga subway". Mississauga Transit routes could be re-designed to a hub-and-spoke operation from the train stations, where you would change to a quick train direct to "downtown" Mississauga, change at Kipling for Etobicoke, change at Dundas West for UofT and the west end, and stay on for Downtown Toronto.
You could get really ambitious and 4-track it, to allow for an express/local combination. Add in more stops at major arterials, and then leave the current stops for the express trains. But that's very much a pipe dream.