Might still be a strategy to scare the interested clubs away from snatching the goalkeeper from their hands, if their game plan is to keep him and not sell him until the one season left until his career expires. Whereas for interested clubs that wants to buy him, I doubt if any of them would want to waste such money, and better not leaving the keeper until his contract expires.
I don't think PSG wants Donnarumma to remain in the club because of some reason which includes his high wages. They have already gotten a replacement, Lucas Chevalier. Maybe they want to make money from his transfer fee. I am sure that many clubs will be scared away until the French outfit reduces the fee. His contract will expire next year, and PSG might not get any amount since he would be free to move to any club he wishes.
I don't know what is happening in the world of football nowadays why will a club like PSG be letting Donnarumma leave the team just because of his high wages like is it this season he started taking that amount? He was instrumental for team I their outing in the champions League competition last season and now they no longer want him because they have a replacement for him already who is probably taking less than his salary, they should reduce his transfer fee so that clubs will rush to sign him but keeping him in the team when he is not going to be used will only reduce his chances of getting a better offer from good clubs.