If Buffon will retire at 65 like he said, we will still have 26 years of his angelic protection. By that time Donnarumma will be 44...It doesn't make any sense to try and sign him.
If it weren't for Mino Raiola, I have a feeling Donnarumma would eventually end up having a Sebastian Frey kinda career. I mean he's good for a 17 year-old but his saves and performances are a bit overrated by the Milanese media. His mistakes are quickly overlooked as well.
Peter Schmeichel was also huge and still was considered a great goalie, I don't think size is the biggest problem for Donnarumma, for starters he'd do well to get rid of Raiola. Besides only Buffon reaches the level of Buffon, at least from the goalie's I've seen over the last ~15 years.