If Donnarumma and PSG don't capitalize on the current transfer window, both will suffer losses. Gigio will likely be benched, potentially for almost a season. For PSG, this represents a financial loss, given that Gigio only has one season left on his contract, meaning they likely won't receive anything in the future. One option is to lower the price; I'm sure Manchester City will try to meet this requirement so they don't have to wait to sell Ederson.
PSG isn't a team that seeks to profit from player sales, so the decision to bench him and his contract expiring next season won't impact the club. What's truly unfortunate is Donnarumma's career, as staying and not getting playing time this season will make things even more difficult. It's better to lower the price so that more teams might be interested in signing Donnarumma, although I don't think PSG's current asking price is too high, considering Donnarumma's abilities as one of the best goalkeepers.
I agree that Donnarumma's asking price isn't excessive, but it's unreasonable to ask for such a high sum for a player whose contract expires at the end of the year. Furthermore, Paris Saint-Germain doesn't seem to be making much of a bid to renew his contract. If it were me, I'd pay him the money he wants and keep him, as he's a goalkeeper whose contribution to the team has always been top-notch.