There is no guarantee that Donnarumma will be able to maintain his fitness like the goalkeepers you mentioned. If he passes 30, we will definitely not get the same performance from him as he is now. Teams usually prefer young goalkeepers like Donnarumma is getting now. If you let this goalkeeper pass 30, then you will see that the demand for this goalkeeper is not very high and it may be that this goalkeeper has to leave for a club outside Europe. I did not say he is old, I only said that considering his age, he may have another three to four years of great performance but after that his performance may become more stable. We want him to play for the Italian national team for a long time but it completely depends on Donnarumma's fitness and his performance.
Every goalkeeper is physically different, but if Donnarumma can maintain his fitness and avoid making dangerous saves, he might still be able to perform well. I agree with you, he maybe need more few years to become top level of his ablity. If he can control his ego and maintain his fitness, I think he can prove himself at the upcoming World Cup. But he also needs a coach and other capable players to play at the top form while playing for the Italian national team and club.