All of what you're saying has to do with Amazon fulfillment (FBA), not Amazon.com the Internet marketplace.
They're two entirely different things (you can actually use Amazon fulfillment for items sold on other websites, although it's typically too expensive). If Amazon fulfillment continues to dominate on an independent platform, that's great, they earned it, but at least they won't have absolute pricing power because sellers will have alternatives, and at least other companies will have a chance to impress you, too.
I'd figure that the marketplace would show the fulfillment company in addition to the way that the courier is typically indicated, so that customers could express their preferences. If the item does a lot of volume, perhaps a seller would even have inventory with multiple fulfillment companies, and buyers could pay a surcharge to get their preferred fulfillment, same as shipping.
They're two entirely different things (you can actually use Amazon fulfillment for items sold on other websites, although it's typically too expensive). If Amazon fulfillment continues to dominate on an independent platform, that's great, they earned it, but at least they won't have absolute pricing power because sellers will have alternatives, and at least other companies will have a chance to impress you, too.
I'd figure that the marketplace would show the fulfillment company in addition to the way that the courier is typically indicated, so that customers could express their preferences. If the item does a lot of volume, perhaps a seller would even have inventory with multiple fulfillment companies, and buyers could pay a surcharge to get their preferred fulfillment, same as shipping.