Doesn't the bitpay service convert the bitcoin to fiat-money automatically for the retailer? So volatility shouldn't be the issue, i think that they just want to stay away from cryptos for now because of other reasons.
AFAIK Steam abruptly ended the partnership with Bitpay due to reasons unknown up to this point. It's not due to volatility since Bitpay handles all the conversions from crypto to fiat in a seamless manner. And no, I don't think stablecoins would have a say/part in e-Commerce knowing that they are basically just tools used to transfer value from one exchange to another and not necessarily as a currency per se. And with what Steam did to the payment option of bitcoin last 3 years ago, I can say that no change of heart has happened even though bitcoin and the whole cryptomarket went to the sky and fell down to where it is right now.