Unfortunately your plan is flawed. The Owners are only interested in maximising profits not winning trophies. If Rashford issued an ultimatum they would sell him and pocket £100m+.
We'll get Rashy to sign an agreement that he won't cave to a bumper contract and will carry out the ultimatum. Plus, it's a different situation. We're far worse off now than when Rooney caused a fuss. Even Rashford wouldn't want to spend his prime years at a sinking ship and they will know that. The club isn't willing to make the necessary changes on its own so it needs to be coerced.