What's stopping someone to doing exactly that in the current system?
All the transactions are controlled by multiple independent parties, whose interests do not align to do that as they’d lose their customers to their competitors when it came out what was happening.
Some of the data does go through central clearing at some step (ACH, Fedwire), but credit card charges, Zelle, cash, checks deposited at the same bank, etc. do not.
With cash, you stay out of those databases in the first place.
Definitely not impossible, but much harder than if there was a single centralized system - dramatically harder.
Cash is king though, as you say.