BOS/NYC are important cities for oneworld broadly speaking. AA has typically been the oneworld partner to offer US domestic connections there, but in recent years they have shifted away from BOS/NYC and focused on their PHL/CLT/MIA/DFW/PHX hubs. We saw this play out with Alaska on the west coast agreement:
https://news.aa.com/news/news-detail...elers-NET-ALP/
And AA attempted a similar, albeit more ambitious, agreement with the NEA and JB. We all know the NEA was shot down, but I suspect this BA codeshare is the start of a copycat agreement like AS has out west. We can't codeshare with AA or AS since our CBA prohibits it, so BA is the best option at the moment.
Jetblue joining oneworld means a massive bank of frequent fliers will now buy tickets on our metal so they can earn/burn frequent flier points. It also gives the TrueBlue loyalty program a huge bump in value, offering more options to earn/burn points on AA, BA, or any oneworld carrier.
Also following the theme of AA focusing elsewhere, we could see a new gate/slot lease deal where we pick up NYC/BOS/DCA real estate from AA and grow those regions even more.