I have never once seen a store that removed the major name brands from the shelves and only offered their own generic brand.
You might argue that having generics on the shelves at all means that new small brands have to pay more for placement, but that's not the same thing as stealing product ideas and leaving no other option but the generic version of that thing on the shelves.
Originally, the generics (which had plain black and white packages) had their own dedicated isles. Now the generics are put on the shelves next to the originals and all those "generic only" shelves got filled with regular products.
I've still never seen a brand new product show up in the store and become popular only to be pulled off the selves and replaced with only a generic version of that product. When I see examples of that happening, I'll accept that grocery stores are guilty of doing what amazon does.