If Toronto wasn’t such a transfer oriented city, and if Yonge wasn’t the trunk it is, then I’d be fine with operating without it. Unfortunately GO is not really being used en masse yet by TTC users so nothing can pick up the slack right now without Yonge. As partially a downtown feeder route, Line 5 will be kinda useless to a lot of people without access to Line 1. Sure you can make circumferential “crosstown” or local trips, but most riders are expecting that after all this time the line may as well do it’s job.
The teething issues of opening a new line are also probably an issue for crosslinx and/or metrolinx; why deal with those while still building the most complex station, and dealing with the public on the lack of Yonge? Just seems like a nightmare for them. I get people want the line open, but opening without Yonge would seal that this is a failure to many in the public, I think. If they take the full year to finish it (I don’t see how that’s possible frankly, but I’m not on the project) then they’ll probably have it looking pristine. Operations will hopefully be mostly sorted out, all stations done, roadwork mostly done, etc. I get the impression metrolinx wants to avoid a further PR nightmare.