Well then it looks like we still have a way to go. Because if this is how it should be "by design", it's not decentralized at all, not even close
Then what you are asking for is impossible. At some point, coders make choices on what they implement that has an impact on the protocol and the network. Furthermore, it is practically impossible to pretend that you can prevent people from arbitrarily choosing to point their clients to another blockchain. Forks will happen because of bugs, developer intervention, and even because a user has decided to make his own personal fork for whatever reason. There's always a game going on about how many people are going to join a given fork.
The only truely "decentralised" system is the universe. You can not roll back anything, time passes and the laws of physics take care of executing themselves with no possible changes on the "blockchain". No coin that is decentralised in that sense can exist. Cryptocoins are simulations of a universe, but humans are one level on top of it, and the protocol can not reach out and prevent somebody from fiddling with the controls. It would be like sort of Skynet world playing God with its own creators.
What he's asking sounds difficult, but I'm not going to say impossible, yet.
There are also laws of human nature, economics, technology, etc.
For example we could use credibility in the human nature economics department, where at a coin launch they simply say "NonDisappearingCoin will have no roll backs". If they did have rollbacks they'd lose brand and credibility and be something other than NonDisappearingCoin. Maybe there can be help from the tech solutions too. I don't know but I'm not giving up till I have irreversible transactions.