Ultimately, it should be pretty obvious we were not getting the OL with TR stock, because it would probably cost at least 1.7-2x as much. Having relief far sooner is better than putting all our money on one line that was never going to fix crowding on Yonge entirely anyways. It is also a legitimately good opportunity to show us that we can build cheaper and faster by going elevated and not insisting on the same rolling stock on every line (very few subway systems use a single rolling stock standard for good reason), there's a reason trains the size of the OL ones have become popular even in massive cities like Shanghai and Seoul.
The OL is not and can not be the last line we build in Toronto and it cannot be the last line we build to downtown. Showing us that we can do more, more quickly and for less is incredibly valuable.