Yes, in that model, they want 'other grocery makers to exist'
But Amazon is going to open a regular grocery store chain soon, so it won't be just Whole Foods.
What Amazon 'wants' is to make money. If they can control all of the food distribution in the US, they will make not only all the profits from that market - they'll make a 'monopolizers surplus' from having the power to charge higher prices in the long run.
That cash-cow is much bigger than the partial cash-cow of owning only an intermediary layer.
