I always love these forums because "everyone" knows what any airline is going to do. Let's wait to see what is going to happen when American or Northwest announces it in writing. Until then, speculation is fine, but please, don't continue to badger your neighbor with the idea that your speculation is right and their's is wrong.
At some point, I would speculate that all airlines will take a look at the 7e7, just as all airlines have taken a look at other projects. However, to say that American will buy the 7e7 to replace the A300s to me does not seem likely as the A300-605Rs are able to carry more freight with the special wings that they have. The B777 cannot replace these aircraft and carry the same amount of frieght, and not even the A330. The A300Rs serve a special niche for American.
At some point, yes, all aircraft get retired, but let me ask this, if American is trying to decrease the number of aircraft types being operated, how will it help to replace the A300 with the 7e7? Swapping one type with another doesn't help. Maybe a swap of B767s and A300s would work, but still, you're then only downsizing by one fleet, as the 7e7 is a new type.
I would speculate that the 7e7 will not be seen in AA
's fleet for quite some time, even a few decades.