I'm not really certain that libra would disrupt much of anything, and there have been doubts in my mind as to whether it would achieve much adoption, unless facebook *forced* it on people, requiring that they use it instead of fiat. I don't think most people have a problem paying for things online with fiat, except the unbanked but there hasn't exactly been a groundswell of people demanding that someone create a stable coin that they can use for...whatever.
Exactly, all other payment methods like PayPal and Visa/Mastercard have a huge network effect, and they can even offer their customers some lucrative bonuses like cashback or miles. Facebook will have a hard time of getting users for their coin if it will ever get released, since they don't offer anything interesting with it - it's bad as cryptocurrency, because it's centralized, it's bad as a payment method, because it isn't convenient and well-adopted.