The one to which you can send back the stuff it sent you, and it essentially burns the money?

And that can’t be fixed because it’s immutable?

Correct, but what they can do is hard fork and encourage users to switch to the new version. Users will vote with their feet and choose the version that benefits them most. In a sense it's still upgradable, but only through direct democracy instead of delegated democracy / oligarchy. (and for the record I don't necessarily think that's a good thing for a financial system, but we'll see how it plays out in the next few years)