I guess that AA will keep investing so that routes that aren’t in the agreement such as US Asia or LATAM will see much better products in future that will also command higher fares. Yes, AY and RJ are now in the agreement as well.
One must remember that one of the benefits to consumers of this is more choice of schedules, especially where connections are required (also applies to the UA/AC/LH/SK tie up or the KL/AF/AZ/DL). As soon as a connection is required – or a multi stop itinerary, it now gives a vast array of out and return options, so you can be booked out on BA and AA back for example which you couldn’t do before so easily. Having looked at flights to Cincinnati recently (BA would clean up if they offered this direct) clients decided on UA as much cheaper in Business Class than BA. Also had other itineraries which were LHR-NYC-YYZ-SFO-LHR and the same missing out NYC and could all be done on UA/AC. 4/5 years ago it would have meant much higher fares to fly direct to YYZ and back from SFO – but now AC and UA are partners it makes it much cheaper as it can be done on 1 ticket.