First, one must assume that most/many/some/alot of DL flights have a few NW/CO flyers on them without any DL status. Those flyers today are NOT part of the pool trying to get upgrades. ANY change would add them to the pool on that flight. In fact, today a NW/CO flyer would generally try to fly NW/CO when possible because they can upgrade there, but not at DL. Change the rules, and more might fly DL. In any event, those that today accept a coach seat now play the seat bingo.

Pretty simple stuff here.
If you are a PM,
you probably would not have a problem with allowing NW/CO folks to also get seats and vice versa. Afterall, you rarely would get bumped out.

Those lowly GM and FO would be AFTER YOU, so WHO CARES?

If you are a GM, you COULD see that last seat go to a NW/CO Plat which would $uck.

If you are FO, you just made it easier for others to get upgrades and to bump you out of line. Your chances would HAVE to go down.