If there are technology challenges, why at least for retail purchases, move away from or better still waste more money investing in technology?

Debit card payments work! If the goal is to add interchange to the ACH model why invent a new set of rails. ISO8583 does much of what ISO20022 does! Again why invent another answer! If we are talking about realtime payroll, P2P and B2B payments and see replacing FedWire as important, then yes let us consider investing in a more efficient and affordable realtime payment solution.