Sheppard West, West of Jane'ish, drops off quite a bit in density and then you'd wind up with an odd terminus for the line at Weston rd. (where Sheppard W ends). If you interline with York extension (Steeles W, York U, Finch W, Sheppard W, possibly Downsview) you'd be linking up 3 potentiall 4 decent trip generators; York U, North York Centre, SCC(hopefully), and potentially the Downview park development area.
This way alternating trains leaving Steeles W would be heading for Sheppard then YUS. YUS trains could then be short turned at Downsview while maintaining high service frequencies south of Steeles W and the Vaughan portion of the YUS line gets it's reduced service one, out of every two or three...