Keeping the same squad will not guarantee the success of the team another time, that's sure since all teams are changing players, coaches, management and tactics. The old players are getting older, there isn't a lot options for using them and giving some changes while making the team younger will guarantee success on the long run eventually. Guardiola know that and he is trying to keep keys players while giving younger ones a chance to play with them.
Using same players used for a season to play the next season is usually not a bad idea because the team will still maintain the same level of team spirit which is very important for a team to achieve success. But most time adding new players to the team is essential, it helps to boost the team morale and create challenges for players in order to improve. Adding new players to replace old ones that are under performing is what all teams does to improve the performance of the team, and mixing younger players with older ones is very helpful as it will boost the confidence of the younger player and help to become more experienced. Young players are as important as older ones because they can learn and develop into more experienced player to takeover from the older ones.