I think it's also unfair to dismiss the Fergus sub in of itself. A GO service would capture any (existing or possible) passengers who currently have to go KW/Cambridge->Aberfoyle->Guelph with a transfer at Aberfoyle, bringing more passengers back onto the rail system and away from motor coaches along the 401 that can get stuck in traffic, especially at rush hours. It would also complete a KW-Cambridge-Guelph triangle of rail services with a completed ION Line 1, and you'd have a pretty seamless transfer. Arguing that passengers won't want to transfer dismisses the fact that existing passengers already have to transfer either at Aberfoyle or in Mississauga, and that's without the existing ION connection. This would go a long way in helping GO (especially post-Greyhound) untangle its far western services and restructure them so they aren't just an attempt to serve 3 distinct cities (K-W-C) with one main bus route (the 25) which has to experience the brunt of both downtown Kitchener and 401 traffic.
The insistence on certain priorities of a single-seat ride to Union once again feels like a consistent bias peculiar to UrbanToronto that isn't in keeping with the repeated Metrolinx studies around the Kitchener line corridor and where passengers actually want to go, as well as latent demand from potential passenger trips left unfulfilled -- much like Metrolinx projected that there was significant off-peak ridership demand at Kitchener and this was borne out in practice. The shutdown of Greyhound has eliminated the one direct connection which existed between KWC and Guelph despite the three being relative neighbours, without the massive sidestep of Aberfoyle. The irony is that even amongst the likes of local news commenters (many of whom are bitterly anti-ION), this news seems to have been received neutrally to positively, and the strongest backlash has been in UrbanToronto and certain railfan Facebook groups, who dismiss the ability for the Fergus sub to potentially carry passengers again, or the utility of a Cambridge-Guelph connection that isn't simply a bridge to get to Toronto. Most commuters in the area are not and will not be heading to Toronto -- just as there are many Kitchener-Cambridge and Kitchener-Guelph commuters there are also many Cambridge-Guelph commuters and vice versa, and most people's commuting patterns are intercity or less, not inter-regional and an hour long or more. For regional rather than inter-regional travellers this isn't "a jog to the left", it helps connect the area. A service like this would alleviate parking demand at Milton, provide the inherent public good of a direct Cambridge-Guelph connection, support ION ridership growth in Cambridge, further decouple intercity transit west of Toronto from the highway system, and support the rebuilding of the Ontario rail network while also preserving the Fergus sub from potential abandonment and redevelopment in the way that's happened with so many branch lines. And it would all be for the price the Region of Waterloo just spent on a new bus garage.