When is a shoulder replacement 100% medically necessary?

Never. The only thing that is 100% medically necessary is something that is life or limb saving. Everything else is about life style and function. If you have pain that you can't treat and poor function, then shoulder replacement is a good option. What you have to ask yourself is whether you can live with things the way they are now? In general, most patients are very happy with their joint replacements.
Rarely. The only absolute indication for a partial or total shoulder replacement would be an unrepairable complex fracture of the upper part of the humerus bone (ball). Other indications are usually for end stage arthritis or a chronic, unrepairable rotator cuff tear that has resulted in severe pain and loss of function. Joint replacement in general is still usually an elective procedure and over 60y/o.
Almost never. The 2 indications for shoulder replacement are cartilage destruction later causing secondary pain. Usually arthritis. Medically necessary surgery is to prevent risk to life or limb. Arthritis is neither. However a shoulder can progressively degenerate making a replacement more difficult in late stages of arthritis.