There's no such thing as a formation that doesn't work. Two things matter: How it is coached, and what players you have.
Regarding how it is coached, the formation comes a distant second to whether you play a pressing game, a deep block, or something else. We have seen that a Glasner team playing (on paper) the same 343 system Sage uses is generally more sound defensively. For his faults, we don't concede most of those goals yesterday under Glasner. He would have us deep and compact, and ensure everyone is clear on their jobs. It would be the same under Roy, or Pulis. Both of those would use a back 4 but that would not be the point. They would play deeper and more compact, and focus on defensive shape. The formation is not the same, nor as important, as the general tactical approach.
We are a mess at the back because we are so open and because the defence seems poorly drilled in any event. If Sage plays a 433 next week but doesn't change those underpinning facts, results will not improve.
The second thing is the players. For me, you should start by listing the best 11 footballers at your club and see whether that list of names lends itself to a particular formation. If it does, bingo. If not, make as few changes to that list as possible before you do find a formation to suit, and use that. In other words, the formation should be dictated by who your best players happen to be, not the other way around.
That doesn't mean you can't work the other way around. Glasner can't have thought Richards was in his best XI as he started with Chalobah instead. Mitchell was always a slightly odd fit at wing-back. In both cases, however, the strength of the system outweighed having a couple of players in the team who were average individually or out of position a bit. It can be done. You can work system-first.
There is surely a limit, though. A huge problem we have is that a lot of our players are very average. There is not a system in the world that can fix that, but surely you don't pick a system that employs more of the average players and less of the good ones? I know he has been 'injured' and all the rest of it but how can a system that includes deeply-struggling youngsters in Khalili and Canvot, the raw and untried Honest, and the wholly out of form Richards in his wrong position be better than one which includes our top scorer from last year? Or pushes Kamada away from midfield after he finally came good playing there to the forward line where he has so little impact?
That's the key for me. 343 is not a 'bad' system. There is no such thing. It is, however, so much the wrong system for what we have got that it is having a negative effect. I get the idea that if three of our centre backs can't defend then what chance will two have, but I see it differently. If all the centre backs are poor then why on earth pick three of them instead of only two?
Ultimately, though, 343 or 433 or whatever else will not matter if it is not accompanied by a viable defensive origination plan.