Q: Should Bitcoin transactions be reversible?
A: Cash transactions are not reversible. If people want reversible transactions, layers can be built on top of bitcoin like credit cards are built on top of cash. Reversible transactions should not be part of the protocol.
Technical addition: If someone wants to change the protocol and fork the client and blockchain, they are welcome to do so and see how many people believe that transactions must be reversible. In fact, this would be welcomed by many to provide people the opportunity to choose.