I’m not sure commingling is as big a problem as you imply. Amazon’s inventory system is random so every item can essentially be tracked by location. Even with commingling, if there is a counterfeit all the inventory from the original sourcer can be tracked and pulled. It’s not like they are randomly tossing all of the same item into a random bin and doesn’t know whose is whose.
Also Amazon buys from many distributors just like every other retailer. Even their own inventory for the same item could come from many different suppliers. Many of those distributors/suppliers are now sellers themselves and all that’s changed is who sets the price and who decides when to send in more inventory (and who takes on the working capital risk.)
As for sellers coming back overnignt, one of the largest complaints on the seller forums is how onorous the KYC process is for new sellers. (Bank documents, passports, business license, etc).
Also you a manufacturer can become a brand owner which prevents commingling and other sellers from listing on their items. Notice that Ankara highest volume power banks pretty much only have offers from Anker and amazon warehouse / woot (an amazon subsidiary).