I personally would not accept that if I was expected to fly long haul monthly. And usually in orgs with such a policy where they say even the CEO flies in Y, in reality the CEO rarely travels so it's a bit silly for that to be a justification. I guess it depends whether they book in full fare Y, or cheapest fare which would block all upgrade options. Also if the travel is often booked last minute the cost difference can be substantial for anything but an AUP.