Apart from Manchester City, other major teams have performed really badly in the last Premier League season. In the next season, those teams will want to build themselves up so that they can correct their mistakes and perform as well as Manchester City.
I don't agree with your assumptions, I think Arsenal and Manchester United had good performances last season, but they can't compete with teams who have a high level of consistency like Manchester City. Arsenal failed to maintain consistency due to the pressure exerted by Manchester City, but that's only normal because Manchester City is the better team.
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If we compare formally, Arsenal scored a solid 84 points, which, although less than Liverpool's phenomenal (for second place) figures of 90 ++ points, but more than, for example, the number of points that Leicester scored in their championship season. Therefore, I also think that many teams showed a good level in the Premier League, but the fact is that City are in a different league and now it is almost impossible to compete with them.