The thing about GO and London isn't that anyone is expecting commuters between London and Toronto, but local commuting from Guelph West in pretty much all combinations of Guelph, Kitchener/Waterloo and London, along with the towns in between has the potential to be significant, is seriously undeserved presently and doesn't even really have a directly parallel expressway. No, full-size train of Bi-Levels aren't ideal here, but this is a real need that isn't going to be served by HSR.
The initial discussion around the HSR project included a Kitchener - London DMU service maintaining local service beyond the end of GO in Kitchener, and this makes a lot of sense, though I've always felt that extending it to Guelph is probably worthwhile for local demand (and that, given the way costs on these things often end up that taking GO all the way to London on the North Mainline, without overlapping service patterns, is probably the best approach is we ever electrify west of Kitchener.