>Transaction reversal is indeed outright harmful to honest vendors, and is only really necessary for the legacy systems because the legacy systems have borderline zero protection against dishonest vendors retaining customers' payment information and pulling money from it

This isn’t the only form of dishonesty. There’s cases where you initiate an honest transaction, the vendor ships you something you didn’t ask for because they are trying to clear inventory, and then refuses to refund or exchange. Then you file a claim with Amex and get your transaction reversed in about a week.