Was the Spadina extension on the subway planned when built to St. George?
I imagine the Wye would not have been done if they knew the line was was going to go north of Bloor in the future.
I think they looked at the situation, and decided to do it.
Same thing with Sheppard, regardless of what was planned, what is the best solution looking at things now.
I suggest interlining is best. It would still be viable if only the extension to Downsview is done. Without interlining, even that would be too short and it would have to be extended from at least STC to Downsview, and it would still need a connection to the Wilson train yard.