Arteta was given a rare opportunity: six months with no pressure on results to work out his best team and his best system. But instead he gave game-time to players who were never going to take him where he wanted to go, and the consequence of that is the mess we're seeing now.
In a way, this reminds me of the end of Mourinho at United (without the rancour). Though there's lots of blame and no need to be sparing with it, ultimately, it's not possible to do any worse with this group of players.
In retrospect, though I get why Arsenal appointed Arteta, when you break it down ... erm, well. He'd never been a manager, he doesn't have the aura of an Arsenal great, and ... being Guardiola's assistant at City is not that similar to dealing with Arsenal all on your bill.
He could still go on to be good – he did really well to win the cup and lots of people seem to rate him – but there aren't many reasons to have thought he'd be good from the off. I'm sure the board will be desperate for him to work out.
It's a madness of football really. Can't be many other arenas in which someone with Arteta's experience gets the top job at an Arsenal-level institution.