Gotta also look at it from the player’s perspective and probably ego as well. When you are the 3rd best player in a team, you expect to start consistently so your overall value goes up even though you might be more useful to the team as a second unit player. It would be hard to keep a player like Monk as a non-starter when it is time for a new contract.
But he's more likely to win a post season award (6th man of the year) than anything he'll do as a starter, he is legitimately playing with one of his best friends, and he's enjoying two of his best years and will get paid handsomely for it (if he can stick it out a third year), or he can go start over and hope the magic carries over.
It depends what his priorities are. But his role is what is working best for the team, so hopefully he's happy in it.