Renforth will attract lots of new riders from central/northern Mississauga switching from Line 2. I expect Cedarvale station will become a major transfer point just from them.
The extension's Wikipedia article says construction is to begin next year at the earliest and may not start until 2029...
https://en.wikipedia.org/wiki/Yonge_North_subway_extension#Stations,_rails_and_systems
Extending the 410 instead would be smarter as there's lots of traffic between Peel and Wasaga/Collingwood, proportionally more than Toronto I'd think, as tourists (and retirees making frequent trips to and from the GTA to visit family) tend to visit and retire to places closer to their sector of...
It was serious and far along in planning actually:
https://transittoronto.ca/subway/5114.shtml
Not to mention the original plan along the Finch hydro corridor.