The EBT cards will never go blank, because no money actually needs to be in the account to give them a balance. Also, it would point blame and anger at the government.
However, after a cycle or two of not being paid, or being paid pennies on the dollar, retailers will cease accepting them.
I think you are exactly correct. The government will likely try to compel food stores to accept pennies on the dollar in return for food and goods.
In fact, there is ample precedent for this with doctors and health care providers being paid pennies on the dollar for Medicaid and Medicare services. Right now this works because of both the health care provider's good will and because they can and do effectively subsidize those losses by charging others (private pay and private insurance)for those costs.
So, I agree, I don't think the EBT cards ever stop working and thereby having people point to the government as the source of the real problem. The government will try to force private businesses to find way to subsidize the force losses from EBT cards. Then the weakest (read: inner city) food stores will go down first and that's where the rioting will start and then expand.