Little sympathy
NW's policy is clearly stated regarding int'l upgrades. Airline FA/ramp workers/reservation clerks/etc - ie every line worker except pilots and mechanics - are not paid a whole lot. One of the reasons they stay with the airlines is the possibility of travel. That's one of their perks. If I or my company buy a business class fare across an ocean, I really don't want other people in the same cabin who bought economy and got upgraded free. If they used miles or paid a bit extra for the upgrade, fine. I don't mind FAs taking up free seats, because they're underpaid to start with and it's a defined perk.
As for the handicap reason, handicap parking spots enable handicapped people with limited mobility a chance at shorter transits to where they're going. I doubt moving handicapped people to the front cabin just because they're handicapped reduces their need for mobility during the flight. As a handicapped person, you must notice the pathetic lazy people who hang a handicap tag on their mirror and practically sprint into their store or wherever they're headed. What if the airlines started a policy of upgrading people because of "handicaps". Can you imaging all the sleazes that would all of a sudden have "handicaps" that magically exist for the duration of their flights?