That's just it, I was going to object to your OP because 'playing exclusively as a centreback' doesn't really make sense for them.
I'd be very disappointed if Song left. We'd lose a lot of presence in midfield that people are keen to ignore in favour of tangential stuff about discipline. We'd also lose continuity and a guy who is almost never injured. Sahin-Arteta-Cazorla is a hypothetical first choice midfield with 12 months of Arsenal history in it. One of those guys goes down, suddenly you're back in limbo raffling places to Rosicky and Diaby in crucial matches.
With the players we've got, we'd absorb it but the flippancy surprises me.