ARM CPUs are believed to be more efficient in the cloud, where operating costs are a significant portion of overall costs. Also, ARM will license its IP to cloud operators, who can then order their CPUs made to their particular specifications. Since they're buying these CPUs directly from the fab (TSMC or Samsung, currently), that also reduces their purchase price.
RISC-V is a future threat for x86 and ARM, not a current one. Well, it's made big inroads into the low-power/embedded market, so it's already hurting ARM there.
Intel could design its own ARM-compatible or RISC-V cores, if it wanted to. I think they probably will, but maybe they're waiting until those markets mature just a bit more. If I had to guess, I'd say they'll skip ARM and go straight to RISC-V.