Westjet's website indicates they have seasonal non-stop service to LAS from some Canadian airports (Regina, Saskatoon, Kelowna, Victoria) that do not have pre-clearance. Will Westjet eventually change to the "C" gates at McCarran; I would expect so.

Thread drift: I remember taking flights in the 70's from YUL to ORD on European carriers who had 5th freedom rights at that time. Those flights did not have U.S. pre-clearance in Montreal; all pax arrived as international arrivals at O'Hare. (Logical - the pax originating in Europe did not have U.S. pre-clearance.)

I believe my first ever 747 flight was on AF between YUL-ORD; I also recall taking flights on BOAC and SAS between YUL-ORD.