The TTC loves building grand terminii even (especially?
) when it has every intention of extending lines further. Even a one year delay in opening the whole extension could be enough for the TTC to say "that's too long for riders to enjoy the wonderful awesomeness of a palatial terminus station...let's upgrade Steeles West to the size of Downsview or Kennedy!." I also hope the full extension is finished before the SE half of Viva Orange is upgraded (for the same reason that the Yonge extension should get going to kill off the Yonge busway).
I really doubt the Finch line will be up and running before 2015...it's not replacing a proposed or serious potential subway project, so there's no incentive to hurry up as there is on Sheppard. I expect a fight once people actually find out about the streetcar ROWs (most people in Toronto are not yet aware of Transit City).