Who else is responsible for the upkeep of the marketplace?
The "break them up" argument works if there is an inherent conflict of interest that would get resolved if they were separate entities, but that isn't true in this case.
The inherent conflicts of interest are everywhere. It commingles inventory, competes with its partners and encourages an auction like atmosphere for constrained product. Retail Amazon and marketplace Amazon need to be segmented for consumers to understand who and how they are buying products.