You're ignoring that Exhibition Station has other attractions at it beside Ontario Place. BMO Field in particular (and the CNE when it occurs), That gives it value as a subway station beyond just Ontario Place, especially when it is extended westward. Also given the ridership of Lakeshore West, that is the most valuable line to connect to in the West. Kitchener/Barrie lines will connect to the 504 at Liberty Village anyway and people can switch to the OL from there at King and Bathurst if they choose to. Yes that's an additional transfer but the connection is still there (or they can take the 504, along it's priority corridor further down King). To connect the OL to the Barrie./Kitchener lines you'd have had to move Liberty Village GO substantially to the east, which means it would be far more redundant with Exhibition/Spadina GO stations, and would also reduce service to people in neighbourhoods like Parkdale who will see more benefit from a station at King/Atlantic than they would at the base of Tecumseh. Plus given the western extension is still to come, and is yet undefined, if the OL turns north, it can always connect to the other GO lines as it travels that way.
Personally, I wouldn't have minded a station under the Ordinance Triangle, since the tunnel passes under it, but the lack of one isn't a disaster, and much like the lack of a station at Cherry, it probably just comes down to tunnel geometry not working to locate a station there, as both of these are locations where the station is heading towards a surface portal, which means either you have to move the portal (which there may not be space to do) or you have to build a station that is literally on an incline, which is insane. So I get why those stations didn't happen, even if in an ideal world they existed.