With what money?How many times do you want to replace a line? Remember, parts of Bloor and Yonge were streetcars. They were upgraded to subways. Imagine if we did that for everything? That would be a waste. Why not future proof the lines?
If the ridership projection is never within any planning horizon likely to approach the required capacity for a subway, is it really "future proofing"?
You really don't seem to understand, there isn't an infinite amount of money that can be spent on transit projects. This isn't about "future proofing", this is about making sure that we get as much transit - subway, LRT, BRT, regular bus, whatever - to as much of the population as possible.
Because of that, the transit needs to be right-sized to the area serving it. Sure, it would be nice to put a subway underneath every single concession - but where will that money come from? Subways cost a humongous amount of money to build and operate, and because of that there really is only a small number of corridors in the City where they can be justified.