ARM has been filling specific server/datacenter roles that Intel and AMD aren't interested in pursuing for for decade(s).
At one end of the spectrum you have Xeon Phi at 288-cores per server and 1.2KW of ridiculously powerful x86 compute performance, RISC servers using high-end ARM designs/OpenPOWER, and of course GPGPU compute clusters using Teslas
At the other end of the spectrum you have Atoms with 16 cores using under 10W, or ARM equivalents if the application is better suited to RISC.
The Xeons and EPYCs really only cover a slice in the middle of the market. It's a pretty big slice but there will always be fringes that Intel and AMD's portfolio of products don't suit particularly well.
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