I don't think it's so much that because Caledon could be served by regional transit. Along with the municipalities spanning the region, its also a matter of north-south routes needing to be split anyways due to length, which the Mississauga/Brampton boundary area is already very suitable for.
If one transit system is created for Halton, that would leave Peel as the only region that still has lower-tier transit. Kind of ironic giving the age and extent of urbanization there, but sort of making sense given its geography of having only two cities on top of one another with no other...
The 410 and part of the 403 isn't actually a replacement of Cawthra, as Cawthra ended at Eglinton, where the concession grid changed. It was built there because development ended at Heart Lake/Tomken in Mississauga and there was open space available to build it to the west, allowing Heart...
Tomken Road was originally the southern continuation of Heart Lake Road, as can be seen with the 410 lining up with it north of Steeles.in the distance.
No seriously. It doesn't exist. And why don't wouldn't you have this for other amalgamated cities where the former municipalities originated as actual towns which are much more visible (e.g: Woodbridge, Stoney Creek), and whose sense of independence is taken a lot more seriously even by non-locals?