Amazon is not just the frontend. There is a whole supply chain side of it with warehouses, distributors, couriers, etc. Inventory and stock readiness, shipping, and other things will continue to be bottlenecks for consumerism at scale.
Did you know the biggest sellers on Amazon normally have their own website and in fact, orders from Amazon flow directly into their OMS (order management system). Amazon is simply a sales channel. Also Amazon has a stronghold in western countries but much less so in gulf states, South Asia, and Africa.
There is no 3PL or other warehouse companies (many of which use Amazon's anyway) there are better than Amazon. Maybe Walmart gets close. For the case you mentioned (OMS integrations), this absolutely does NOT mean that they handle shipping. The aggressive majority of those vendors just uses FBA anyway.
If you're building your startup on the principle that Amazon is "a sales channel only", then I wish you well but you will fail.