Don’t let the title fool you: I am still in favour of LRT over most every subway expansion proposal. I am fine with extending Yonge up to Steeles. I also suspect that there may be some benefit of extending Sheppard east to Victoria Park and having the Sheppard LRT in the Transit City proposal meet it there (although, that would eliminate a connection with the Don Mills LRT).
While I am not 100% convinced that either the Vaughan or the Yonge extensions are done deals yet, betting on either going forward might be a reasonably safe bet. That said, some comments on other postings here have raised some ideas about more useful ways to expand the subway system, if subway expansion is in the cards. Check out the post by Andy MJ on The GTA Patriot.
I personally feel that LRT should be implemented from Downsview to York U and on to Vaughan, where it would connect with future YRT/VIVA LRT operations. This would also connect with the Finch LRT and possibly also with the Jane LRT proposed under Transit City. It is these connections that make a rapid transit network - and it is a network that we need, not a skeleton consisting of a backbone and a couple of ribs that we have to funnel people from a wide area onto.
That said, if a subway must go to York U, I am becoming more of a supporter of extending the Sheppard subway to that destination. It would provide a useful cross-connection between the two subway lines at the north end and, at the same time, significantly increase the usefulness of the Sheppard line while providing people in the east a convenient way to York U.