I believe OP is speaking specifically to the IoT space, which is still a bit of a stretch - but slightly more accurate than the massive ARM dominance we're seeing elsewhere.
RISC-V is a much better ISA for IoT since it's so much better aligned. You just need to use the parts that you want instead of having to put in the whole package. Plus, it's free!
ST, Atmel and Microchip maintain massive product catalogues so users can choose between 4MHz, 10MHz, 16MHz and 20MHz for their 8-bit, 8-pin microcontroller, and choose whether they want 1750 bytes of program memory or 3500 - but for the same price customers can get a 160 MHz, 32-bit microcontroller with built in wifi?
Why would expressif pay the engineering costs and supply chain complexity costs to make worse products, when their flagship product is already a great price?
It's not about the number of ARM cores shipping today, but tomorrow. If RISC-V tools are about as good as ARM and the cores are cheaper it will become the preferred instruction set somewhere in the future.
Seems in this age of open source tools the instruction set is mostly commodity anyway.