Surely I can't get unlimited free groceries by just always paying with a credit card and then doing a chargeback, right? Like, it might work a couple times, but then wouldn't they just close my account?

I have no idea. Probably depends on how big you are as a company that the credit cards would care enough to step in rather than lose a customer or how much that customer costs them.

All I know is that for the chargebacks I've gotten for my saas, I submit a mountain of evidence that the user used it heavily and then filed a chargeback after the subscription ends.

Most of the times paypal will even agree the customer scammed, and they say they will talk to the credit card company on our behalf. However we have never once won one of these chargebacks if it was done with a credit card. Then paypal proceeds to remove the money from our account and tack on a chargeback fee.