Arteta doesn't have good backup players when the squad feels tired or injured, of course Arteta doesn't have other players to rely on, even though Arsenal could have won the league this season, unfortunately Arteta didn't really manage their players at the end of the season, Arsenal have been around for so long top of the table before but unfortunately at the end of the season they looked tired and inconsistent.
Arteta tried in the just ended season but Arteta those not have much experience that was what stopped him from winning league for Arsenal, how will their be a team and you don’t have back up, toward the ending of the season, from Arsenal performance drop, you will know that those players are tired, how will you keep on using the same squad from the beginning of season till ending, they will get tired and that was what happened to Arsenal, Arteta tried but if am to blame anyone why Arsenal didn’t win the league, then am still going to blame Arteta for that. But am sure he will correct his mistakes before the next season will start, but if he those not, then Arsenal won’t really perform well next season.
This was the problem that made Arsenal unable to win the trophy due to lack of back up like you said. Another problem is the players are too young and lack experience on how to tackle big teams when they are on pressure. This was the only opportunity that Arsenal had to lift up the trophy again after so many years but lack of preparation in the long term run messed up their fate.
I believe that Arteta will learn from his mistakes and buy more players which will help the club when some are tired to keep the consistency of the club because Arteta is a good coach,if he could manage these players from the beginning of the season to the end and they became second on the table.