Every time we have this discussion someone brings up FedNow, and I will repeat the same question I always ask: when I visit the farmer's market this weekend, will anyone there be able to practically accept payment in FedNow? What would that even look like? (FYI the vendors take most cards, Apple/Google Pay, Venmo, paper cash, Square Cash, Apple Cash, etc.)
If the answer is "no for these reasons", then this probably shines a big light on why FedNow is not serving the same use case.
What is preventing any of those mentioned card vendors from integrating with FedNow either directly or via some abstractive layer through another entity? I don't understand why the answer would be 'no for these reasons'.
The retail payment companies I've seen all use the same structure: they provide a retail interface and then handle monetary transfers within their own proprietary network (effectively a centralized database). To interface with the financial system, they provide a mechanism to occasionally wire funds to/from a traditional bank account. If FedNow has any role in these systems, it's just to speed up the occasional funds-wiring process by a few hours. I have yet to see anyone actually directly using FedNow in any meaningful sense for retail payments.
Most likely, what it would look like is they would have a routing and account number posted. You'd go into your bank app and push a payment to those numbers, and they'd say yeah great; not confirm the transaction and everything would probably work out.
Is that satisfying? Not really. Is it possible? Yes.
There are over a thousand different companies affiliated with FedNow, so the answer is going to be "it won't look like FedNow, but you will use some wrapper for it"