Steen is way more fragile than Backes. Backes was out of lineup due to diverticulitis, not because his body is falling apart like Steen.
They're different players to begin with, but the argument is his style of game is what this team needs more. Backes still puts up over 200 hits per year, and close to 200 shots on goal per year. Steen only had 117 SOG last year. And Backes' Corsi possession numbers were much stronger than Steen's last year.
It's all about building a team and IMO your team needs a little bit of everything, it needs speed, skill and some toughness and guys that are willing to pay the price at the net. And to me, bringing back your Captain wouldn't of been a bad move for Armstrong and he would of hardly been blamed for doing so. Furthermore, you still had control of Steen and could of gotten assets in return for him. I'd rather have Backes who fits a need here plus additional assets for Steen than just have Steen and nothing else which is what we got.