Do we have any evidence of this? Walmart still sells non-great value products (started in 1993!) while I get why copying a niche product might cause the original manufacturer to die out and if there are examples of this I am more than willing to change my opinion.

If you actually take a look at their offering (https://www.amazon.ca/-/en/stores/page/BFAE3282-E15B-47EE-90...), most of these are legitimately "basics", things that existed 50-100 years ago. Wilson won't die out because Amazon sells a volleyball, nor will Duracell because of offbrand AA batteries