Using the existing right of way is problematic because it belongs to CN. HS trains and freight trains can't use the same tracks - North American freight trains would destroy high speed tracks. So the corridor would have to be 4 tracks, which is wider than the corridor will allow through the urban areas. If HSR takes over the corridor altogether, a new corridor would have to be built for freight, which defeats the purpose of using an existing ROW.
The other issue is going through small urban areas like Georgetown and Acton, and even Guelph. It's very unlikely that any of these places would get a stop (likely one high speed station will serve the whole K-W/Cambridge/Guelph area), so you'll either have 300 km/h trains roaring through these communities or trains slowing down so much through every community that they'll never get up to speed. The former would cause huge disruption to the communities, especially Guelph where it goes through downtown, and the fight they'd put up would make the Weston battle look like a cakewalk. The latter would defeat the purpose of HSR altogether.
The last HSR study contemplated using existing rights of way where CN and CP lines run parallel and could be consolidated, and where one of the two is actually suitable for high speed, ie relatively straight and not running through any significant towns. Like between London and Chatham.