Regarding Bailly: it's difficult to make a proper judgement on him due to his injury record being rubbish. He seems to pick up injuries and niggles ridiculously easily, which is not what you want from a centre back. Imo, your centre backs (actually, no, your entire back five) should rarely be changing since you want them to develop some sort of understanding with each other. You want them to instinctively know where each other are so that they can drop back together, push up together, know who is marking who on set pieces, know who has who covered on crosses, etc. There needs to be an almost-telepathic type of link between them.
Whichever defender comes in next needs to have an exemplary injury record, imo. Need to be available for near enough every game. We have to stop chopping and changing every week based on who is injured and who isn't. Form is a bit different, however. I was one of those who was happy that we had Maguire and Lindelöf playing together most weeks, for the simple fact that we've given them both a go and given that partnership a chance of being fruitful. If it doesn't work out in the long-term, at least we can confidently say that it's been tested and it has or hasn't worked.
I'm not willing to give the Bailly experiment another try. He's had four years at the club, which is plenty of time to establish himself. He hasn't done so. Needs to be moved on and somebody else needs to be brought in to have their chance. If we keep Bailly for another year, we will be in the exact same position as we are now, only with him being a year older. It's about time this club were more ruthless, as top clubs should be.