FedNow will allow instant bank transfers. That means that you can build a payment system on top of it similar to Windcave Account2Account (that we have here in Australia and New Zealand)

Windcave Account2Account is not a full replacement for debit/credit cards. It does not have PayWave (solvable). It has a clunky UX (solvable). It does not do credit transactions (not solvable).

It can be a minor headwind to VISA/Mastercard and slightly reduce their new signups and tx volume, but I cannot see it fully replacing credit/debit cards.