Ahh i wasn't thinking about it as a consequence of some hard battles with injures that are so fatal that you lose your arm.
But decisions you make and options you take.
You give something up and get something else.

(unless a consequence of a hard boss fight, he does die but tries to take something away from you)(((reminds me of the naruto scene with Sarutobi vs orichimaru)))

It is indeed very weird and very optional. It could be even restricted to a class, to make this class more intresting. If, for whatever reason your fighter/monk run seems to boring. There would be an option to spice it up abit.
A deal with the devil and you lose your arm by magical means with no way to recover it, but getting an artifical replacement.
Just one of many ideas i have and this one wanted to be spilled out.