A microcontroller is a means to an end. When I need a CPU for an FPGA design, I will always use RISC-V because there's plenty of easy to integrate open source CPUs available. Similarly, when I need a hard microcontroller chip or board, I use whatever suits me best. And then I frankly don't care whether it's RISC-V or ARM. Why should I? I use what's available and has the right features.

There's nothing irrelevant about an RPI Pico: it has more RAM than most, it has excellent documentation and example code, it has PIOs that are incredibly versatile, it's available everywhere, and it's dirt cheap.

For quick prototyping I use MicroPython, otherwise I use C. Why should I have to learn Rust for something simple?