You are more right than you know. The hospital absolutely will be willing to coast by with locums for years. They don't care. It is very unlikely you will get a pay raise by threatening to resign and likely may end up with the opposite outcome. It is very possible that the hospital has already determined they are paying you above market rate and intends to cut your pay when your contract is up and if you resign, they will accept it and not let you take it back.
I was told (explicitly by the CEO) that it was preferable to lose a rad onc than it was to lose RTTs, dosi, or physics, and they weren't lying. It is far easier to get locums than to make an employed rad onc happy by replacing or correcting insubordinate or incompetent staff (especially in a "not great location"), and the locums doesn't push RTT, dosi, and physics (because they don't care), and then the complaints go away. I have watched it happen. Admin's solution to this problem is to tell the staff they are right and the rad onc is wrong and make the rad onc so miserable that he voluntarily resigns, then bring locums in and try to start over with a more passive new hire.