1. Toronto made its position clear back in 2009. YNSE won't move forward without the Relief Line moving forward first.
2. Yonge Line will be significantly over capacity (1,300) with 15 min SmartTrack. It's only under capacity with 5 min SmartTrack (33,000) at TTC fare, but we already know that will be impossible. Headways will probably be at about 9 minutes, it will probably have a premium fare, and won't provide adequate Yonge relief.
3. 15 min SmartTrack and the Relief Line Short (to Danforth) would divert enough riders to bring the Yonge Line just at capacity (only 400 below capacity). They'll be no room to spare for the additional 2000 riders from YRNS. So the Relief Line will need to be extended north to at least Eglinton before we can talk about YRNS. Heck, even bringing it to Eglinton might not even be enough to divert another 2000 peak hour riders, since the only route it diverts from the Yonge Line is the Eglinton Line and a few minor bus routes.
5. These ridership projections do not take into account the planned Crosstown West and Crosstown East, which will add additional pressure onto Yonge Line.
In conclusion, York Region is out of their minds. They'd better start planning their own solution for Yonge Street transit, because the Yonge North extension isn't coming anytime soon.