To spell it out, the merchant pays fees to the payment processor and carries risk (chargebacks, etc) and these costs are included (“embedded”) in the purchase price of whatever you’re buying.
Moving to instant pay moves these risks to the purchaser, which is probably not ideal for the merchant because it forces purchasers to be more careful with their spending. New merchants in particular would have to work harder to establish their reputation. Larger merchants would probably start offering credit again.
Where “insta pay” shines is for merchants with less credit worthy customers, because it allows them to operate online and in an electronic world. Currently in the U.S. that job is done with cash, but perhaps very soon with privately issued stablecoins. I guess the big question is whether the U.S. government should issue a stablecoin or similar electronic cash-like thing.