Updated - since my text was confusing. The subject at hand is subsidized grocery stores, the USSR is an example of failed centrally planned subsidized grocery stores.
The USSR is an example of many failed things. That they failed at those things does not mean those things cannot be done.
I was offering a tangible example of where the thing being proposed was a failure.
They are examples of the USSR failing at a possible thing. They illustrate my critique of your claim.