Amazon bought Twitch for its video live streaming infrastructure/tech, which at the time was unmatched. Now Amazon offers that infra/tech via AWS and anyone can spin up a Twitch competitor just as capable. Amazon doesn’t care about Twitch at all. The Prime sub thing is just another extra they tacked on to make Amazon Prime more appealing to consumers, but iirc Twitch is the one who actually pays out of pocket for Prime subs.