I dont think the point Amazon uses ARM was about performance but purely cost optimisation. At one point, nearly 40% of Intel's server revenue was coming from Amazon. They just figure it out at their scale it would be cheaper to do it themselves.

But I am purely guessing ARM has risen their price per core so it makes less financial sense to do a yearly update on CPU. They are also going into Server CPU business meaning they now have some incentives to keep it all to themselves. Which makes the Nvidia moves really smart as they decided to go for the ISA licences and do it by themselves.