I like the first phase, and it makes too much sense. An extension to STC is unlikely to occur under current plans, considering the Line 2 extension is going to Sheppard, which sets up that intersection for an interchange between the lines at Sheppard-McCowan, but it's serving pretty much the same place, so no problem.
As for running the Sheppard subway to the airport, I am not so sure. The Finch LRT just to the north is going to the airport, and sure, it's slower, but consider that Line 5 will also be going to the airport just to the south. Line 5's western arm is going to be entirely grade-separated and essentially a subway from Yonge-Eglinton area to YYZ; the time differences won't be significant. Between Line 5 and Finch LRT, there wouldn't be ridership demanding a subway sandwiched between them.