Lets think about this objective. The wireless carrier market is roughly 260 Billion dollars large.
They buy boost mobile and they defacto get access to all the certification processes etc of how carriers certify phones and how carriers really run their network. I dont think amazon wants to run a network. I think Amazon wants to tap into the 5G market emerging on both the consumer and professional end. I think this in particular is not a win for consumers despite having another player on the market. Amazon is currently the largest online retailer. This is just another move to control the entirety of retail from consumer to professional.