Well that's one more airline who won't be ordering 797/MOM planes as 767 replacements, then.
Now this isn't bad news for Boeing at all - they are buying 787s instead, after all. It doesn't mean that AA will never order 797/MOM at all either, just that the 767 replacement part of the plane's mission is getting less and less significant as the months pass without a launch date.
That's quite the leap. If the MOM ever exists and is cheaper to operate on TATL than a 787-8 by a significant margin, why couldn't they buy that for the TATL routes and move the 787's to other routes? Just because the 787 is more cost effective than a 767 doesn't mean that it will prevent any future order for a model optimized for these routes.
But if the earliest a MOM could possibly go into quantity production is 2025/26, which seems to be the consensus here, and AA intends to replace all its 767s by 2021, then any future AA order for MOM planes won't be as 767 replacements, will it? It may be because AA believes the new model is ideal for some of its routes, including TATL, but it won't be because there is a pressing need to replace 767s, since there will be no 767s by then.
Instead, if they do eventually come to believe that a 788 is too much plane for TATL work (which remains to be seen), they'll have to find something decidedly profitable for those 788s to do instead that allows them to justify the capital expenditure of buying MOMs for these routes. Organic growth would probably take care of that over time, but not overnight.
I'm not sure this analysis is entirely correct, due to the confounding factor of the LUS A330 fleet, which is now scheduled to stick around longer than the LAA 767 fleet. Perhaps not quite long enough to get to 797 production, but close enough to not be ruled out. True, AA doesn't have enough A330s now to do the TATL flying they would like to, thus the staging of 767s at PHL in the summer to cover the new seasonal Central Europe routes (and a few others), and this shortfall will grow as the PHL hub gets built up further. On the other hand, if AA downgauges some ex-PHL Western Europe flights to A321neo/A321LR, that can free up some widebody frames for further TATL growth.
What is probably a more compelling argument is that, between its heavy investments in both the A321 and the 787, AA has the smallest possible space in which it could ever justify a separate MOM frame, and thus the best possible business case in the industry for skipping the 797 entirely.
Speaking anecdotally, local PHL travelers do not appreciate getting the unupdated 767, it feels very much like a merger-mediated hand-me-down. Even LUS 757s are more loved.