I'm not so sure. Unless people check that there is an existing situation before leaving home (assuming central Toronto) before committing to go 400 or 404, it seems most regular weekend travellers have their route and seldom deviate from it. I used to run from Barrie to Pearson and on weekends would go nowhere near Hwy 400 but instead use a route of Sideroad 15 (CR 53) through the marsh to Weston Rd. Never stopped (well, except for stop signs). In a few places you could see Hwy 400 and it would invariably be stopped.
If it is to be a bypass, be a bypass. If it were just an upgrade or linking of existing municipal roads, there would still be the issue of private driveways, cyclists (don't know if they will be prohibited from the bypass), etc. Besides, if that's all they were wanting to do, the Region could have done it years ago.
I'm not convinced the MTO sees filling in the numbers as a pressing problem.