What do you mean by maintaining the old ones? Do you mean players that are already there or players that are old in terms of their age?
I think there can be a good reason to try to maintain the squad over the course of several seasons if possible. The top teams mostly had a certain core of players that played for the same club for a longer period of time. The list of examples is long. Real Madrid, Manchester City, Barcelona for a very long time, Bayern Munich. It is no coincidence that they tried to maintain their core players for a very long time while adding some salt to the soup over time with new players or some top stars if available. But yes at the moment Arsenal is on a good way, but before we praise them too much, I prefer to see how they finish this season.
What I mean is maintaining only the previous squad without signing new players, those teams you mentioned though retained their previous players but you can't tell me that they didn't make any signing within those years so that's actually what am talking about. Okay for example, Leicester city, after they won the premier and still maintain same players without adding much signing instead went ahead to sell some of their players what happened to them? Where they active again as compared to when they won the league? Obviously not!
Arsenal's fate is still swinging yet as their possibility of winning either the league title or the Champions league isn't certain yet but I can assure you that these boys are eager for a title this season that was why they flopped and at a time they picked form again so it wouldn't be a surprise if they take home a trophy this season.
No you are mostly right with what you just posted here. Leicester City is a great example, but there is one thing that I believe should be accepted as a matter of fact and is called a miracle because it is a miracle: when a team like Leicester City wins the Premier League (and this is a great example by the way), there is essentially no chance that this can be repeated. That's why these events are called miracles. They lead the table for 6 weeks, 12 weeks, then suddenly 18 weeks and still everybody thinks that they must crash soon and they still benefit from other teams underestimating their potential consistency. Then Leicester City kept the lead for over half a year and that was when the big names became slowly nervous. In the subsequent season, I don't know what Leicester City could have to realistically have a chance to defend the title. It was obvious they wouldn't be able to di it with the same team because that was a once in a lifetime thing. But at the same time they could have never signed three or four superstars because superstars wouldn't go there despite the fact that they won the Premier League.
I am curious to see what's going to happen to Leverkusen next season. This season it looks as if nobody can stop them and then the question would be legit to ask whether they are unstoppable for years to come. But no, Leverkusen will run into problems as well. Maybe not as hard as your example (Leicester City), but Leverkusen has momentum now and they are using it perfectly. That momentum will be gone by the beginning of next season. They will start from zero and no team in the Bundesliga will underestimate them.