There's plenty of players out there who can improve them but they never seem interested. Why don't they go for someone like Krychowiak or N'Zonzi? Someone who can sit deep and defend, Xhaka was supposed to be that man and he looked class in Germany but he's been underwhelming so far. They could also do with a no nonsense CB, Kos is a very good CB but he's not someone who will constantly stick his head in the danger area and is prone to mistakes.
Too many of their players have stagnated on their 'potential' but still constantly get minutes. Ox, Walcott, Welbeck, Ramsey, Bellerin, Gibbs, Coquelin are all decent players who have had spells where they looked top players but ultimately they are not good enough to be starting every game, you can have 1/2 potentially to do a job but having 4/5/6 starting every week is not going to work if they want to challenge for the title.