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By looking at the situation that has happened to Arsenal in the last few seasons I think the problem is in terms of their mentality. We can see that for now their coach is actually still quite capable, the players are also still above average quality although there are still some things that need to be improved but for Arsenal at this time they are quite complex and able to provide more.
But when their mentality remains the same then in the end the same situation every year will not be inevitable even though their performance is very good at the beginning of the season but slowly they collapse and we have seen this for Arsenal several times in the last few seasons where those who were even dominant and successful in securing the beginning and middle of the season were destroyed and could not do anything to maintain their dominance at the end of the season.
There you have it, you gave the answer yourself in your post. The players they have are above average quality. I agree that they are and above average quality is never enough to win the Premier League. You can't just play with above average and hope everyone else plays average. Liverpool plays highest quality, Manchester City usually does, many other teams have the quality to do it at least sometimes. Arsenal needs more top quality players.