I think the development of a coin is not out of the question, but the incentive isn'tn there yet, since the adoption of coins is not yet really at the level of making it a huge profit item for them, especially when you consider that they have their Credit Card, which makes it much more attractive to them to marjet and push that, and not a coin.
The driving forcer will be the inertia and acceptance of coins and tokens being inevitable, and they will likely adopt a model where they take a few of the most popular coins, to get that huge segmentof the market to use Amazon Prime and be part of the many7 tentacled beast Amazon would love to become.