Canadian here. Supply management is probably great for many producers. They participate in legal cartels engaged in mandated price-fixing, so this can protect their bottom line. For consumers it keeps prices high on things like dairy (even with the CAD in rough shape, I could still find cheaper cheese just across the border, in the US; pre-covid, mind you).

Most major countries have some kind of scheme in place to fix agriculture prices. In the case of the US there are subsidies that everyone pays, via taxes.