This. The 77E fleet (47 units) will fly on for a while longer still, and were overhauled in 2013-2014, but a better and more likely replacement for them is an improved 787-10, with more range, improved MTOW, and more cargo capability. I think that's a more likely scenario than the 787-9. AA has a big network and so can and should be able to fill a larger jet at multiple hubs, like the -10. I don't see AA going for an Airbus wide body like the A350 (reviving an order that began with USAirways and was cancelled). Operational simplicity seems to be a higher focus for AA and I'd think they would get a good deal on new Boeing jets anyway.
I have a different angle. I don't believe in AA will get B787-10.
What I think is, the additional order of B789 few years back was the initial process to replace some B77E (probably 10-15 frames). Then A330s were retired early, so plan changed.
The move of make a batch of B789 into 51J and then B77W into 70J, makes me wonder when AA makes next jet order, we may see 20-25 B777-8 or B777-9 in next decade
. If we look at the situation of B77W, it worked very well for AA on some routes. B787-10 can not replace the capacity of B77W, and B77W won't be in AA fleet forever, at some time (in another 10-15 years) AA will have to replace the fleet with compatible frames. Using BA's B787-10 as a yard stick (as BA use Club Suite which is similar to AA's J seats), B787-10 is likely to be 250-260 seat aircraft for AA (assuming 50J+, if 60J or 70J the seat count would be as low as 220). It is still less than B77W.
My thinking is B787-10 may provide capacity lift from B789, but it can't replace the B77W, based on the assumption AA's next widebody order would have B77W replacement in mind. Plus, B787-10 if ordered, can only support LHR+HND (summer) and SYD +GRU + EZE (winter), with possible CDG/MAD/ATH/BCN in summer and TLV as well. So it means if B787-10 is ordered it will be a small fleet like B77W.
It is important for AA to achieve fleet simplicity; I fully agree with you. So I dont know if B787-10 could help the case here. Rather, I think a top up order for B789 is more likely in the next 5 years. Also, with A321XLR coming, AA's appetite for long haul jets may be less than we thought. A321XLR can release some B788, then further these B788 can free up some B77E for retirement. It won't be 1-1 replacement, but by 2025-2026, we will have a completely different picture of the AA long haul network. Hence I believe some of the B77E might be retired without replacement as the capacity will be channeled to A321XLR routes. So B787-10 might not be needed after all.