De Bruyne is still one of the best midfielders. I don't know why Guardiola let go of De Bruyne. De Bruyne is 33 years old. He could have served City well for at least 2 more seasons.
He has been one of best attacking midfielders in many seasons during last decade but his team is counting down. Naturally no players can win against ageing and when they are young, they lack of skills and experience while when they are old, they have good skills and abundant experience but their bodies are no longer strong and fast enough for physical fights.
Kevin De Bruyne with his experience and excellent skills, can play very cleverly and it is helpful for him to save energy, avoid direct physical fights as well as maximize his contribution. Anyway, it only helps him to cover ageing effects until he no longer be able to do this, and surely that time will come, not only with De Bruyne but also with all players.
De Bruyne faced injury in the 2023-24 season. He played only 18 matches. Even then, he scored four goals and 10 assists. Last season, De Bruyne played 28 matches in the Premier League. This midfielder scored 4 goals and 7 assists.
With time, season by season, he will become more vulnerable to injuries and will get more, then have less appearances for his club. At least Napoli can still take advantage of De Bruyne's contributions in next one or two seasons but for longer time, it's very uncertain and unpredictable.