There are other x86-64 CPU manufacturers, but you're making a massive trade off for performance. Intel and AMD really have polished their stuff until it shines.

If Amazon can prove ARM can cope with server scale operations, and even outperform x86-64, you can have a high degree of confidence that other companies will get in the game. There are many more companies manufacturing ARM chips than x86-64 (due to licensing agreements, in part)