Amazon doesn't have to compete with AMD on performance, only on TCO. Not having to pay Jim Keller's salary is a good way to reduce your TCO. Graviton generally does win on TCO for most applications today.

Also, if 30% of x86 market share corresponds to ~20% of "big processor" market share, that means that non-x86 cores have about 30% market share.

European companies are discovering the same thing, too. They have embraced ARM and RISC-V very strongly.