They are not only taking up all the first page organic results, but they are all taking up the first page advertising spots too + Google Shopping on the first page.
I was just thinking the same. Best what ever happend to big brands. Now they are able to overtake the complete search engine for ecom terms. All the small site that are much more cheaper are vanished. Donīt know if this is a good UE to the users. My niche is domiated by 2(!) big brands ( and Amazon ) this two brands have nearly 80% presence on first page! This is a upgoing spiral for brands and their signals, hard to break through. And a Multi Million Dollar spender like Amazon will not pay the CPC you have to pay!