They don’t need the pile of cash, and are free to burn it if they want and just write down in an excel sheet: “today we burned 2 billion euros”, now the question is if you’re willing to have your salary paid out in a digital recognition that you now own a part of that burnt cash, because you feel comfortable that others would allow you to pay using those IOU’s of no longer existing cash.

If not, then the current system where the digital records refer to actual cash still needs there to be actual cash somewhere. Normally you’d have it in the national bank, but since they started charging it’s better to stockpile paper.