I don't really agree with this point of view. I would accept it, if you mentioned high fees, but although price volatility might seem to be a problem, and it would be if the change was to rapid, I think that most merchants could just sell bitcoins as they get them. So they would just need a script to keep the prices accurate, regarding the latest changes, and then convert the coin back to fiat as soon as they got them. I'm sure there are many services from the major wallets, that would provide that service to merchants.
For instance, when you deposit bitcoin in neteller, you will get an exchange rate, and you will get fiat in your account. That service, that in this case is provided by bitpay if I'm not mistaken, could easily be used by any merchants, so the only problem I see are the fees and not the volatility.
Of course that right now, not even the fees seem to be a real problem, because segwit adoption is increasing, and LN is being developed at good pace, so things will get even better.