You are going to pay him Buffon wages at 18 years old? In couple of seasons he will demand Messi wages. 3.5M euros after taxes is the highest wage in Milan and he has rejected that. And as it stands we cannot compete with the Manchesters or Real and Barca when it comes to wages. So it's definitely not on the club.
Would I pay the most promising GK in the world, at 18, the same as the 39 year old world class goalkeeper? Probably yes. Think about the Messi salaries when and if that happens.
I'm not sure what you suggest. The only way become a great club again is to have great players. The only way to keep a player like Donnarumma is to offer competitive salaries. He is doing exactly what anyone else would do: this is a life-changing opportunity to sign a contract worth more money than all members of this forum combined have ever touched. At 18 years old. Or he could stay at Milan after the first offer not piss off fans, and risk breaking a leg two weeks later, change of manager, new signing, never again that kind of offer. If he goes to Manchester City because of an extra 250 grand a year, by all means criticize him, but if they offer to double his salary and we don't get close to that, while we are spending dozens of millions of dollars on Kessié and Musacchio and rumors have us spending hundreds of millions, then it is obviously on the club.
I wish it was not like that, but it is. It's not only that people seem to be holding a kid like Donnarumma to the standard of the likes of the players who spent their whole career in one club. It's also that these players did not stay at their clubs only out of "love" and "loyalty". People like Del Piero, Gerrard, Totti, and Maldini often had massive contracts and tremendous incentives to stay at their clubs (as they competed for trophies or played in excellent leagues). They also were part of several hot transfer rumors when their contracts were close to expiring. It did not happen when they were 18 most of times, but they also were not the youngest goalkeeper in the league's history, etc.
Now, you can say Donnarumma is just not good enough to bet on that: cash in on him and buy a solid keeper and perhaps even more players with the money. You might be right. I would personally bet on him becoming a legend. The potential is there. But there clearly is a choice -- the kid has never said he wants to leave. Don't put it as if what he was doing was ridiculous or the club had no choice, or, even worst, in moral terms.