Isn't there is wildly higher ridership on Yonge between Highway 7 and Finch now than there is traveling from Scarborough Centre to Don Mills without growth? At STC there are significantly more people headed to downtown than North York, and at Langstaff the bulk of the traffic heads south on Yonge. It makes no sense at STC to provide more capacity to Don Mills than to Kennedy.
Rush hour AM peak buses on Yonge, between Highway 7 and Finch:
Viva Blue - every 4 minutes
Viva Blue A - approx every 15 minutes
Viva Pink - approx every 15 minutes
99 - every 12 minutes
Rush hour AM peak buses on Sheppard east of Don Mills:
85 - 15 buses/hour (about every 4 minutes)
190 - 10 buses/hour (about every 6 minutes)
I am not counting the YRT express routes (300,301,302,303), 98E (runs in PM rush hour only), GO buses along Yonge St, or bus routes that only run part of the way from Highway 7 to Finch (for Yonge: TTC 42, 53, 60, 125, YRT 2, 5, 23, 77, 88, 91; for Sheppard: TTC 167, 224).
If you look at Yonge between Finch and Steeles ridership is clearly much higher than any part of Sheppard east of Don Mills, but north of Steeles and especially north of Royal Orchard ridership drops considerably. Ridership on Yonge north of Royal Orchard might be a bit higher than Sheppard due to the use of articulated buses, but I don't think the difference is enormous. On the other hand, ridership on parallel routes to Sheppard (Finch East and York Mills) is much higher than ridership on Bathurst and Bayview in York Region, so we could see people switching to the Sheppard subway from these routes.
Also I find it hard to believe that if we start to see massive redevelopment at Yonge/Highway 7 replacing the big box stores (10000s of residents and jobs), that we won't see similar massive redevelopment at Downsview, NYCC, Consumers Road and STC. This simply doesn't make much sense.