In order to replace the registers you’d probably have to replace the entire infrastructure. You’d run into compatibility issues if you didn’t. Just look at the ipads for example. They were brand new devices that were still bottlenecked by old hardware and the software that had to also be compatible with the old registers.
So if you replace the registers you’d have to replace the wiring and hookups for it. Then you’d have to replace the mainframe within the stores. Then you’d have to replace everything else.
Technology isn’t compatible with everything. You can’t just swap out one piece of a complex system from 30+ years ago and expect it to work with something made recently.