AC appears to be trying to ape more dedicated international carriers like VS and BA (and others, but those are the only two I have experience with); it'll be interesting to see how the idea of Premium Economy plays on intra-Canada routes, where there's pretty fierce competition from WestJet and Porter (again, among others).

UA might be convinced to take E+ upmarket if they see there's a domestic market for it, but I don't see them adding it fleetwide otherwise. (Or unless DL beefs up their YComfort offering.)