I haven't seen Maxwell play at all Sergio - decreased production is not a great sign - but I haven't seen the guy play, I just have no idea what he has to offer (in terms of skillset, or situational strengths).
Beyond that, all I will say (and what you certainly know) is that obviously stats don't always tell the story. Has he been shifted between positions, who has he played with in those 3 years (linemates), did he miss time with injury (if so, what type of injuries were they), have the coaches asked him to significantly adjust his role, etc., etc.
Anyway, I'm sure he probably is in decline, and, like I said, the coaches/scouts in the organization seem to be down on him for one reason or another.
A change of scenery might be the best thing for everyone involved. I still like this trade for Atlanta; they clear salary, move a surplus dman, and a minor leaguer in exchange for a prospect who's still only 22, and a 4th rnd pick. It's a good deal for them.
You never know when, or if these guys will put it all together, but from ATL's perspective, 40 points in 47 games isn't the worst place to start. I'm sure he pretty much replaces Dawes production in the AHL at the very least (probably for less money), and Sopel for a 4th seems decent enough for a 34 year old who's out of contract after this season (I think), and has been a healthy scratch lately.