This has been said before, and it should be said every single time this idea comes up. Monroe's Bird rights do not transfer with him. Accordingly, to sign him in the off-season would require we have the cap space to do so. Unless the Pistons are willing to take on Wallace's contract in return, we are unlikely to have the requisite cap space to offer him a max deal. Even if the Pistons take on Wallace, are likely faced with a choice of 2 of Rondo, Monroe, and Green.
So here are the questions: Is it worth a 1st rounder having Monroe for 2/3 of a season and risking that you lose him to a team with enough cap room willing to outbid you? And if you think no team will outbid you, is it really worth a 1st round pick to get a guy for 2/3 of a season when you could just sign him next summer anyway if you really want?
Again, this isn't "he walks at the end of the season because he doesn't like it in Boston". This is "he walks at the end of the season because another team can and wants to offer him millions more per year." To me, it's not worth the risk, because it's likely Monroe will go to whomever offers him the most money, and it's unlikely the Celtics can be that team.