I'm guessing his body feels good, he still has fun playing, and he wanted to call when he goes out. It's really not a money thing, and I think he knows that as a whole, he's been well-paid for his career. He's never going to be able to play competitively again after he retires, at least at the level he's at now, so he probably wants to leave with no doubts. Now, if he starts the season 10 for 75 and his shoulder aches, then maybe he packs it in in May or June, but if he feels good and productive, he'll be happy.
Q for me becomes, if the NL uses a DH, is the regular line up Bell at DH and RZ at 1st, with SWLABR every day in left? That would give the Nats a second nice righty threat beyond Turner and balance Soto / Schwarber / Bell (who hits like a lefty).