Bringing in new players does not guarantee that next season a team's performance will immediately improve, but what must be realized is that it is a step that must be taken for other possibilities, the top Premier League teams have prepared to improve their strength to cover next season and of course no one knows for sure what will happen but at least there will be changes that occur when a team makes changes.
Buying new players is always a roll of dice. If the players you buy are good and they fit your system then you will become much better, and if the players you buy turn out to be not fit your team and play bad then you will become worse. We have seen this plenty of times in plenty of seasons, it's always an unknown until it is not. So we need to consider how this will go with time, it's not going to beknown beforehand.
Liverpool, Arsenal, City, these are all great teams and we have no idea how they will play until they end up playing. This is why we need to give it some time. By week 10 or so, you will see clearly which team is doing how. Obviously we can see before that too, but week 10 makes it guaranteed for everyone. At that point, only injuries change teams, otherwise they are what they are all year.