One possible scenario, for example: convenience is key. So people shift from cooking at home(only ~10% love to cook ) to buying fresh, restaurant quality, heat-and-eat meals.
Key parameters to compete here are quality, selection and price. Walmart could offer good quality via help of external chefs, if they get critical mass they could offer enough selection, and since they buy much more food than Amazon ingredients will come cheaper for them, supply chain will be cheaper for them, and and the only place Amazon may have an advantage is more efficient food making process, so there some chance Amazon could win where(altougj most like the innovation will come from a startup, which also Walmart can acquire) , but even so ,will it negate all of Walmart's advantages, and won't be copyable in some form ?
And remember, even for wealthy people that care less about price, better cost efficiencies could mean better food, better reviews or better paid marketing.
So What about other Amazon differentiators: Amazon Ecbo ? Google will offer that. Instead of just delivering stuff, putting them is people's fridges. Is that something that people do? Is that something so hard to copy ? And is that really a meaningful differentiation ?