I beg to differ on that one - AC uses wide bodies on both regional and longer haul domestic routes. AC and UA have a similar number of 787s and AC has committed to retiring a lot of the older aircraft they have (i.e. A330 and 767) in favour of an all Boeing fleet. Also if you've ever set foot in an AC 787 it's easy to see that their J cabin is better than what's on offer from UA on their non-Polaris 777/787s (i.e. no neighbours).

Here are a bunch of examples of AC "domestic" routes using wide body lie flats:
YYZ -> YUL (1 hr flight):
https://www.google.com/flights/#sear...;sc=b;s=0;eo=e

YUL -> YVR (~ hr flight):
https://www.google.com/flights/#sear...;sc=b;s=0;eo=e

YYZ -> YVR (~5 hr flight):
https://www.google.com/flights/#sear...;sc=b;s=1;eo=e

YYZ -> YHZ (~ 2 hr flight):
https://www.google.com/flights/#sear...;sc=b;s=0;eo=e

YYZ -> YEG (~ 3 1/2 hr flight):
https://www.google.com/flights/#sear...;sc=b;s=0;eo=e

YYZ -> YOW (~1 hr flight):
https://www.google.com/flights/#sear...;sc=b;s=0;eo=e

I can go on and on with additional routings but my head would spin!

Safe Travels,

James