Zelle is owned by a consortium of the largest US banks [1], and does much more volume than Venmo (because its built into the UX of those participating in Zelle's real time payment network [2]). Congress strongly encouraged the Fed to develop FedNow to prevent Zelle from keeping smaller banks out of instant payment infra. FedNow will eventually replace the ACH system.
Hot take: banks being banks, they'll crowd out fintechs because fintechs aren't chartered banks regardless of the UX (which is why Bancorp Bank is the underlying for so many fintechs).
[1] https://techcrunch.com/2017/06/12/zelle-the-real-time-venmo-...