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.