Personally I co-run a rescue with 17 pasture puffs, each with their own series of issues making them unusable by the masses. But none of them are currently at the point where they need to be put down.
If she is able to stand and walk and eat and graze without pain or trauma there is no reason she can't be a pasture puff.
IMO there are many times you need to put down an animal, when their suffering is too much, when they themselves have truly given up, when you and no one else can afford to prolong their difficult life - BUT it is not acceptable to put down any animal just because they are inconvenient. Seems to me like she needs you in her life. :)