My mom had surgery in March and they wanted her to fill out a living will. She had already made me in charge in case anything should happen, so I told her to just leave it at that and not fill out the living will. The living will said they could take her off life support after so many days, yada, yada, yada. Personally, I don’t think you are in a position to make those kinds of decisions beforehand. There are too many unknowns.
You really can’t blame a hospital for wanting something in writing. If you aren’t in a position to make those decisions before hand you sure as heck won’t be in a better position when it comes down to life and death crunch time.