I think the idea is just that ideally a player like Quincey is kept or replaced by a similar player or upgraded by a player with a similar impact and a guy like say Smith is replaced by Russo or whoever.
I think there is a validity to the idea of maintaining certain roles. I agree that if a guy is a net positive impact on the ice he should be there, regardless of how big/ small, offensive/ defensive, etc he might be. Karlsson is one of the most valuable D in the league and he isn't running guys over or "shutting them down" in a defensive sense.
But at the same time, it seems like almost none of the defenders currently on this team enjoy battling for pucks, especially along the boards, and have a lot of trouble boxing guys out with their bodies. Russo would not help the team in this regard, though he might be able to put together a more polished passing/ transition game than someone like Smith if things go right for him. Where as guys like Ericsson and Quincey are two of the few D who look like they can body a guy out of the crease or off the puck along the boards if it came to it, despite how rarely it actually happens, so people will value them because of that.