My take on this is that the Graviton stuff exists to solve the Catch-22 problem: AWS wants you running ARM stuff, so they need to provide an ARM platform for you to do it on. Current suppliers may not be providing anything cloudy enough from the enterprise management side of things, so AWS might have to roll their own for now.
Once the ball is rolling, they can get 3rd party ARM chips or full servers in and stop with their own development on that side if they want to.
Amazon has been putting their own chips into systems for years, after they bought Annapurna Labs - they have hardware virtualisation support in their network and storage chips, and now their own CPU. This does not seem to be a short-term tactic.
Once the ball is rolling, they can get 3rd party ARM chips or full servers in and stop with their own development on that side if they want to.