I dunno if Cortex-M chips will be better for long, though. Some of the RISC-V specifications like interrupt controllers need a bit more time, but they have very good support in mainline compilers like the GNU toolchain.
My experience writing C for RISC-V MCUs is about the same as doing so for Cortex-M platforms. They're a bit low-volume/expensive atm, but the core is very appealing to chip makers because it has solid software support and no royalties.
I also wouldn't write ARM off, though. Their 'coming-soon' MCU neural accelerators could open a lot of doors if it works well on a low power budget.
That seems to be said of a lot of things that do end up disappearing. Hard to tell whether there are real roots in the market or if it's going to evaporate like so many other things.
They spent the last 10 years beating out PICs, 8051s, etc. Because they were BETTER.
And now a million engineers now how to use them, and the tooling from 1000+ companies is all in place to support them.
They aren't going anywhere.