ARM is special in that it's the only potential, realistic competitor to x86 left in the entire industry. RISC-V? Only if you're a zealot breathing fumes for life energy, at least as things stand today.
The interesting thing about RISC-V is that there are like 10 billion RISC-V microcontrollers out there that otherwise would have been ARM. So ARM has been moving into the PC/server space while RISC-V pushes in from behind, at the opposite end of the line from x86.
RISC-V is disrupting ARM's low end, the pace it's occupied safely for decades. As RISC-V matures, it will move upstream along the same path that ARM did. The difference is that it will be easier to move because the transition to ARM is demonstrating that companies don't need to be locked to a particular ISA.
