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.
Just putting Donnarumma on the bench is also not beneficial for PSG, because his contract expires next summer and they would simply lose him as a free agent. Yes, you can say that PSG has a lot of money and they are not trying to sell players for profit, but Donnarumma is a very expensive player, and losing him as a free agent doesn’t mean losing 5 or 10 million, we’re talking about tens of millions.
And if I’m not mistaken, right now they are trying to agree with Manchester City on a price of 35 million. That’s why, of course, it’s better for them to sell this player now, and I don’t think it makes sense for them to keep him on the bench for the whole season just to prevent some team from getting a good goalkeeper.