There will always be pros and cons regarding player transfers, especially for players who have just helped their club win a trophy. Many may not know the background that made Donnarumma leave PSG.
Regardless of whether the coach is satisfied with his performance or not, I believe Donnarumma has given his best. If PSG treated him poorly, I think there are other issues that are definitely not disclosed to the public. What circulates might just be speculation from the media.
Honestly Donnarumma has given his best to the team I really don't know why the coach don't want him anymore but if he's made his final decision then there's nothing Donnarumma can do than to leave to a team where he'll be relevant, welcomed and given playing time. I wish he leaves than decide to stay and be on the bench.