And yet, if you go into a store and buy something with cash, you have never been able to just arbitrarily reverse that payment and keep the goods several months later, and the sky has not fallen.

Humans doing stuff make mistakes, and need to be able to "take it back."

And what about the humans who didn't make a mistake, but simply chose to abuse the system at the expense of whoever they transferred the money to? Certainty and security are important for those on both sides of a transaction. If you have a dispute, that's what legal systems are there for. The quasi-judicial role that financial services play in resolving disputes, often with little regulatory oversight and absolutely no requirement to be neutral or objective, is long overdue for retirement.