BNPL is often free to the customer [1], to whom it amounts to a free loan. The merchant, however, is soaked for the interest/fees. So merchant prices will rise slightly, for everyone, to cover those costs..... much like credit cards do now. Cash and debit users subsidize this scheme without getting the free loan.

The downside for the customer is that it promotes overspending, which could be argued as a net negative on the whole. A careful consumer could make use of the free loan, investing the cash elsewhere during the loan period.

[1] https://www.bitsaboutmoney.com/archive/buy-now-pay-later/