I've often wondered why NW/KL haven't tried to expand more aggressively their partnership with CO. If you look at their strengths and weaknesses in terms of routes, they seem to compliment each other well. NW & CO have said off and on that they would like to form a fourth major N. American route system, but have stopped short of truly providing that ability, I think, by not enhancing their partnership further.
I think building this partnership would be great for all involved. I don't know whats holding them back.
BTW: I wouldn't say CO's inflight service is particularly great, not any better than KL for instance. Their biggest strength is their Bethune lead management team that took them from 'worst to first'.