I think you and Oliver are being a bit glib tbh.
It's not like I've watched Conte, Klopp, and Guardiola (who is flexible, btw...he sometimes uses 3, sometimes 4...that's kind of the point, a strong tactical coach can do this, as Pochettino showed as well...it doesn't have to be something so unrealistic as you guys are making it out, to use it occasionally when it is helpful, as it would be in this case, given we are without any RB whatsoever of decent standard) and said: aha! that's the light, that's the new way of football, let's go to 3 at the back always! No; I've seen us have an injury in a position where we have no backup, I've seen that we have, on the other hand, have decent depth at centre back, and I've just said a simple and pragmatic solution.
I was a strong believer in possession football long before Barça-prime btw
Formations for me are meh, I guess if I'm a fan of one it's 4-3-3/4-5-1 because 4-4-2 tends to be more for long ball football/English football with traditional wingers, but in general I like the one that fits the players best. Given that we don't have a RB, and we have a good ball-playing CB on the bench, and we're currently devoting a central midfield spot who contributes less to our play with the ball than said CB (or Mustafi), a 3/5 at the back would seem to be logical whilst Bellerín is out. It won't happen because Wenger isn't a technically flexible manager (which wouldn't really be a critique if his 'plan A' produced good football--indeed, it was a critique I defended him against until I realised there just weren't any excuses left for the lack of quality of the plan A), but just because the only manager we've known for the last 20 years is tactically unflexible, doesn't mean that when you look around and have some perspective it's really anything but a pretty simple ask of a manager.