A lot of software stacks are still only optimised for ARM. TrustZone is also far more mature and supported than the RISC-V equivalents.
(FWIW I agree with the other commenter that these ""security"" features are useless, and feel to me more like check-box compliance than anything else (Why does TrustZone work with function calls? What’s wrong with IPC! Also, what’s wrong with privileged mode?). Just seems like a bit of a waste of silicon really.)
esp-c6 module parts are like $2-3 and already have an fcc cert, low power, and support all the same protocols I believe. Are nrf54 modules really that cheap? Can I program them natively with Rust yet?