[skydhash]

That would be fine if the dependencies were permanent. Hardware fail and need to be replaced. The OS will be upgraded (macOS is more than happy to make breaking changes). If the software is networked, that’s another transient plane. Libraries will fall out of support range.

Then there’s the fact that the user’s needs fluctuate. Imagine having to pay for a whole another software because the current code is spaghetti and full of hardcoded value and magic constants. It worked, but now you want a slight adjustment, but that can’t no longer be made unless you’re willing to rewrite the whole thing (and pay for it). That would be like having to buy a whole new car, because you moved to the house next door, as the car is hardwired to move only between your old place and where you work.

[carlosjobim]

In my opinion, the OS should not be updated. Not if important software is running on the machine. That's why we see cash registers still using Windows XP.

Sure, if you test it and see that there is no issue with updating, then you can update if you want. But neither the OS or the hardware or anything else should get any priority over the business-crucial software you are running. Even with hardware failures, the better option is to get older hardware for replacement if newer hardware has compatibility issues.