There are unpatched 0-days that go back years that still make WinXP dangerous to have on any sort of network. Not only that, but they will never be fixed. Nor can you fix them yourself (no source code).
And yes, cash registers will be networked because of data mining. I can get an accurate picture of store utilization solely by watching registers. And also by seeing what was purchased, I can change inventory appropriately. So yes, networking is essential. Perhaps it's not for the small business that handles flea markets and such.