I don't know what Bailly has done to earn this cloak of invincibility. He's had a season to forget with a number of seriously poor games either side of the injury, but he's somehow untouchable, despite being able to barely header the ball. Smalling plays nearly every game, plays rather well for the most part, and most importantly he's been incredibly reliable, but he's first on the chopping block.
I think he could be improved on if we paid big money for a proven defender, but that shouldn't cast him out of the team either just push him down the pecking order, with the others making way. Right now he's above Bailly on both performance and reliability, and it's not even close. Even Lindelof had a better season, but no matter what, it's always Bailly+1.