well first consider those characters are real people(jonah's character was renamed). Billy Beane has stayed in oakland all these years because as he said to Peter "I really wanted to win here" Here being the key word. He wanted(and still wants) the challenge of it. Plus by going to Boston he would have been doing it for the money a mistake he already made and had to live with once. He also wanted to remain close to his daughter. He's also loyal.

peter brand is really Paul DePodesta who later would go on to be the GM of the Dodgers and now works for the Mets as an assistant of some sort. You don't go from being a guys assistant one year to being a GM the next esp when a huge portion of baseball people are anti-sabermetrics

the movie ended how that season really ended. and to be clear Billy did accept the red sox offer at first in real life but quickly declined it because it didn't feel right. and of course Theo epstien took Beane's method to build a team and they went on to win a ws