WF has already served Amazon well by being a test bed for grocery delivery optimization, no need to screw up a profitable existing business with any additional radical changes. That's what Amazon Go stores are for, and now they can sell that tech to other stores. Once the tech is mass-adopted, they can smoothly switch all WF stores to "just walk out" tech with very little complaints, as that's the experience people would be already used to at all the other stores
Not only that, Amazon would be literally paid by others to take over Amazon's own risk. This is such a nice play all around, they just need to manage not screwing it up somehow. But execution never was a weakness for Amazon imo, so I bet it will mostly go smoothly.
If I was a major business, I wouldn't pay Amazon to be a guinea pig, especially when I see them unwilling to do it at their own major retail chain.