Those fridges are from the 90s. You cant reasonably support it that long. It's not even just software. The PLC is fixed hardware, you can't simply patch that.

You can replace the PLC. It's not like a modern PLC couldn't be made to control a 1990s fridge. The question is "Who will pay for the upgrade". DanFoss sure as hell won't. The stores won't, because they've already paid for the fridge.

Honestly I feel like this should fall under a support contract. The pricing structure could be similar to that of Microsoft. Support is fairly cheap, X number of years included in the purchase price, once the product reaches EOL the price goes up dramatically each year.

There's a massive problem in how we think about these products collectively. They are inherently insecure, they will be attacked and preventing it is expensive. You could build a safer alternative, but you'd be priced out of the market instantly by less safe, but more convenient alternatives.