Using existing bus routes as a barometer for subway demand is flawed, but something I see on these forums all the time. If I was a York U student living around Warden or Kennedy today, I would not consider taking a Sheppard bus to Don Mills, transfer to subway, then take another (brutal) transfer at Yonge to get back to the bus bay to take the express bus to school. I would just drive (unless I was poor, in which case I am not a 'choice' rider). Even when the Spadina line completes, that would not make me change my mind.
If I had a Sheppard subway I could take from Scarborough that required only one transfer at Downsview to get to York, that is a *different* story.
I think the Sheppard west extension particularly has a lot of merit. I'm sure something like 95% of people who get off at Yonge/Sheppard station from the Sheppard line just transfer to the southbound Yonge line (helping to overcrowd it), even if their final destination is St. George (UofT), or St. Patrick station (hospitals). If the subway would continue on to Downsview, I'm sure many of these riders would prefer that and transfer there.