It's used widely in Chinese stuff (which is basically everything) so in terms of volume it's probably already dominant.
In terms of dollar volume ARM is still the leader, especially for higher-end (application level MCUs) stuff. RISC-V MCUs with MMUs or MPUs are scarce at the moment.
> RISC-V MCUs with MMUs or MPUs
MPUs are more common. The Physical Memory Protection option is basic and allows ranges of memory to be set unavailable in user mode. A few well-defined ranges do let you lock a user process down securely but it's not a real MMU. Low-end microcontrollers don't have enough RAM to warrant a real MMU.
The RISC-V core in the Raspberry Pi RP2350 has PMP as do the ESP32 cores.