The problem is you cannot safely have both unreliability and immutability. To get reliability you have to fork. In fact you may have to fork every time there is a bug in any smart contract which meets a certain market cap threshold. This is the only way for Ethereum currently to shut down a runaway process on their world computer.

If you have reliable smart contracts then they'll never have a bug. So you never would end up in a situation where a smart contract is literally out of control and owned by no one or worse owned by an unknown attacker. Maybe in the future smart contracts will be better designed with built in mechanism to allow external shut down by an arbitrator but what if that has a bug and fails too?