Those lines aren't getting transfers from an elevated GO line less than a 7 minute trip from Union Station, which has an alternative connection (Yonge Line) available.The conclusion that deep tunnels would reduce ridership by 15% certainly isn’t consistent with my extensive experience of London’s Central and Piccadilly lines,...
TTC, when designing the RL they asked GO if they could use a bit of corridor and got told absolutely not (they eliminated those options without any evaluation as a result). They designed around that limitation. Metrolinx is now stating that restriction they put on the RL would severely harm their own goal on getting passengers off GO trains before Union.
I understand why they made that decision at the time; RER hadn't been spec'd out but probably needed corridor space and the underground Lake Shore line station was the preferred Union relief mechanism; but we could have been flushing out these details in 2015 rather than today with a cooperative rather than combative Metrolinx.