Why would a vendor use something that has variable value? I don't see any crypto beigg very useful in transactions that are intended to compete with fiat. You can't have a crypto that competes with fiat, otherwise regulation would destroy the ability for the crypto to be mainstream.

What crypto is good for is really 1 thing, validity (hence the immutable ledger aspect being the golden cradle crypto). How we can use validity to bolster the value of a crypto coin is the question I think we want to answer.