The issue with ACH isn't the batch nature, it's that banks on both sides of the transaction are allowed a fairly long amount of time (upwards of a business day) to decide whether to accept or decline the transaction.
If they decide immediately, it can go faster, but they don't have to, and many banks have no incentive to work on processing faster.
They could still use a batch processing system, and just process the batches once per hour, or half hour, or whatever. It would still be slower than some newly-designed instant payments system, but could potentially be more palatable within the current system.
ACH is used either to get paid by an employee or to move money between accounts, neither of those things are that easy to do with Venmo, in fact zelle was created to compete with Venmo and it addresses that competition so nicely that many of my friends have moved to it.
If they decide immediately, it can go faster, but they don't have to, and many banks have no incentive to work on processing faster.
They could still use a batch processing system, and just process the batches once per hour, or half hour, or whatever. It would still be slower than some newly-designed instant payments system, but could potentially be more palatable within the current system.