u-Bob

That's the point. You can't say that someone's life is worth living because "he's still able to play a jigsaw puzzle" or "still able to wipe his own ass".

The question is not if someone is still able to perform certain functions. The question is if someone is allowed to determine how to live his own life (as long as he does not cause damage to another human or his property of course).

Only the person in question can decide if his life is worth living. Only he can decide if he still sees value in his life. Only he can decide if he's willing to live with the pain or limitations he's burdened with.

You can of course disagree with his decision, but you can't force your idea of what is a "life worth living" onto him.