I know you're making a point but there is a distinct difference in risk: FedNow transactions happen instantly, therefore fraud can take place faster.
That's all though. That's not a good reason to stay on FedACH.
For the HN crowd the best way to describe the difference between FedACH and FedNow is migrating from a batch-based system that settled transactions a few times/day to a real-time system. Just like such migrations developers and engineers encounter in many normal IT environments, real-time systems have their own issues but are also capable of so much more and are generally easier to improve/evolve.
FedACH was made for big, one-time transfers between bank accounts where it wouldn't matter too much if settlement took a few days. FedNow was made for lots of small(er) instantaneous transactions.
At first FedNow will likely be used as a simple replacement for ACH transfers but I suspect that it will eventually replace the back ends that handle debit card payments (because the prize--which would be taken from Visa, MasterCard, First Data, and similar--is too big to ignore).
