We've been discussing the reason that hasn't happened yet for years. Because there isn't (or at least pre-Covid wasn't) enough available capacity on Line 1, and especially Bloor-Yonge station, to handle the resulting increase in ridership.
So there's very good reasons that it hasn't gone first, after phase 1 of Sheppard. But that that the Relief line didn't get started around the time of Sheppard is where the zero sense (zero cents?

) was.
This is why that even of the 4 big projects announced around 2019 (Ontario, Line 2 to Sheppard, Line 5 to Renfrew, and Line 1 to Richmond Hill), that the Yonge extension has been the last one to get going - so that the Ontario line opens first.