IMO it depends on who ends up being responsible for fraud and insufficient funds. It sounds like FedNow will provide liquidity for instant settlement but the page doesn't make it clear what happens if the sender somehow has a insufficient balance. If a $500 FedNow transaction occurs and a $500 ACH transfer settle on the same day (and the account only has $500), who get's paid and who made the mistake?

With ACH most banks/financial services provide some amount of instant funds but wait 1-3 days for the transfer to complete before providing the full balance. This is very useful since it gives the bank 1-3 days to be alerted of fraud, insufficient funds issues, etc. While ACH can technically be clawed back after 3 days, it's a much more difficult process which mitigates risk by forcing the sender through a lengthy fraud claim process.

hopefully! for now it will be up to financial institutions to implement the service and pass on the savings.