That they'd probably have to retain is one of the keys here. Any team that he'd be willing to waive to probably is up against the cap so retention would be needed. 50% retention would be $3.2 million/yr for two years. So the 'cost' of a backup for Dostal is already $3.2 before factoring whatever the new backup is going to make. Even league minimum pushes things to $4 million (and a league minimum goalie means that Dostal will be getting a LOT of the work - potentially burning him out like they did with Gibson for a few years). If they've got to spend a couple million for a goalie probably inferior to Gibson, they're better off just keeping him and have no worries about goaltending for a couple of years (again, this is assuming that he has not demanded a trade).The Ducks have the most cap space in the league. In fact, they will likely ben under the floor if they got out of his contract.
Moving him just to get out of his contract would be the dumbest thing ever. Especially considering he is barely over the league average for goalie contracts, and they will have to retain/take back cap.
And all of that is before we even get into what kind of return will they get for him. Goalies generally don't return much for whatever reason. If all they're getting is a couple of mid round picks (and a 1st from a serious contender would be a very late 1st most likely) or a lower quality roster player (something the Ducks really have no need for), we're back to 'why bother trading him in the first place if all it will do is make the team worse'.
Hopefully PV sticks to his 'the cost will be high' guns otherwise the Ducks will likely badly lose any trade for Gibson.