Calgary needs an elite #1 centre (which Bennett isn't going to get), a true top 6 RS RW (which Calgary would probably need to add to acquire) or prospects/picks. They don't need LW's, middle/bottom 6 centres or cap dumps. If they had room for a middle 6 centre they would just play Bennett as the 3rd line centre and everyone would be happy.
I'm not sure Mantha is better than Dube or Mangiapane other than him being a RS. Calgary can't afford to be paying a 2-3 line RW 5.7M until 2023-24. Mantha is a good player but not a great fit that is worth giving up one of our best prospects.
"But you get to dump Lucic"
Lucic is overpaid but is playing well, Helm is garbage. Lucic is having a clear influence on our young players giving them confidence, if you watch the games you'll see the effect he's had on Andersson and Dube for sure. Also, since Lucic joined the team no one is taking liberties with our players anymore, no 21 slashes to Gaudreau's wrists in 1 game. He may not fight a lot but he is the nuclear deterrant.
That's why Flames fans (at least me) aren't crazy about this trade.
Edit: Oops, wrong Bennett thread.