Could be but i'm not as concerned.
I think what has to be understood about Coleman, and why he is different than other FA targets the Flames have signed, is how well rounded he is. This is not a player who can only score goals (Brouwer, Neal) and he isn't a player like Lucic, who even in his prime was not fleet of foot. Coleman has speed and is a very good skater, impacts the game in all 3 zones, can play special teams (more pk than PP) and still is a good shooter. I think this is a player that even if his offense starts to dry up in 3-4 years you will still likely like because of how intelligent he is, he can still impact the other parts of the game at a very high level even if he is not scoring.
In terms of his scoring he is a very consistent 5 on 5 player. Whether or not he can be a 45+ point guy likely would come down to PP time and utilization but his scoring rates at 5 on 5 and very similar to Backlund and over the last 3 seasons his goal scoring rate at 5on 5 would put him 70th in the league.
I get the counter argument that you are betting against father time and father time is mostly undefeated. However, at the AAV they gave him I feel it's a really reasonable bet for the Flames even at the term they gave him because of how solid a player Coleman is.