I think you're being perhaps a tad extreme on this, though. He definitely was not excellent down the stretch, but few were. And not giving him responsibilty clearly did not improve the team anyway.
There really isn't a defensive specialist who reliably shuts down first liners in the NHL. It's not a thing. They're first liners for a reason. And it's only 1 player. Occasionally, you get together a line of specialists that does a great job, but even that is not commonplace. The worst power play in history and I find it hard to blame the checking line of cycle guys -- at least one of whom was basically a randomly acquired spare -- for not shutting down the most talented players in the world. It seems very odd. Quite honestly, our 5 on 5 differential was absolutely fine anyway. You can win with that.
I think he'll do fine in a 3rd line role. He's absolutely good enough.