Well, if people decide to use it that way nobody can stop them. In my opinion, that would be an inefficient way of using the network, since if you pay with XLM

Well, if people decide to use it that way nobody can stop them. In my opinion, that would be an inefficient way of using the network, since if you pay with XLM today you need to first buy some, taking a loss on the conversion since you have to go through either eth or btc, then the payee needs to sell it (again with another intermediate currency) to get the money to pay their operating costs, and they'll likely charge a processing fee for that, and at that point you may as well be using a credit card. On the other hand, if you use it like intended by getting USD stellar tokens from an anchor, and paying with those, the merchant can more or less accept them at face value (if they trust the anchor to give them USD for the token)