Donnarumma is undoubtedly the best goalkeeper and if the list of the five best goalkeepers in the world is made now, then Donnarumma will definitely make his place in that list. However, the bad news for Italy is that this goalkeeper may not be able to provide good service for much longer because he is already old enough. In the selection of goalkeepers, different clubs or different teams always give more importance to young footballers, in that case, in two to three years, this goalkeeper may not be in the same form as before. I wonder what will happen to the Italian team when most of the players who are currently playing for the Italian national team retire. Italy may continue to play football, but we may not see good performances from them.
What do you mean that Donnarumma is already old enough? He is 26 years old, he was born in 1999, the other teams still play with players like Courtois (1992), Oblak (1993), Neuer (1986), Maignan (1995), Pickford (1994), Alisson (1992), just to mention a few, so I really have no idea how you can say that a 26 years old goalkeeper is old. Buffon played until he was 40+ years old.
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.