Something's going on here. Justin's always been a pretty quiet dude, he does the work and goes home, I can't imagine him just quitting without giving the fans a proper sendoff.
That said, if this was just him saying "I'm done", I can't blame him. He's nearly 50, he's a millionaire several times over by now surely and he's...bored. Alex, unlike a lot of the other characters, only sporadically has something interesting to do. And frankly he looks bored in the majority of his scene because he has nothing to do.