the main reason ARM CPUs are so popular is not only their low power usuage, but also the eco system around the AXI bus. AXI bus is used inside the SOC to connect third party modules to ARM CPUs, you can easily build a custom SOC by plugging bunch of AXI capable modules together with an ARM CPU.
unless RISC-V builds a similar bus on a chip to AXI with a bridge to connect to legacy AXI modules, the RISC-V platform will go no where