Supermarkets operate on razor thin margins. They rely on being smart and avoiding work to make a profit. I don't think they are going to be persuaded to do a bunch of extra manual work, and tolerate increased electricity bills, just on the off chance that there's a once-in-a-generation cyber attack.
I think, in classic HN style, you're massively underestimating the complexity of a domain that you have no experience with. There's a whole academic literature on optimizing this:
https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=supe...
The software running on fridges should, obviously, be secure (just like any other software should), but air gapping the fridges, and winding the technological clock back a couple of decades, isn't a realistic proposal.