It is a choice at the software later. And who is making that choice? It's the initial people who actually write that software.

But what if that system is now affecting many other people, or the entire planet in a significant way? Should they have some voice over that?

Under a traditional governance regime the answer is that that is at least possible to change. It may be difficult, but it does not violate the laws of physics and can happen in less time than the heat death of the universe. But we can now write software that makes it functionally impossible for anyone to make that choice, even potentially the original designers. That is an option that we did not have before. It's in some sense the essence of trustlessness.

In some cases, this might be the right trade-off. For example, beyond the blockchain, this is also a way to think about encrypted communications. It is a very significant new power that we can now wield.

But it must be wielded carefully, and that doesn't seem to be happening.

So yes, blockchain applications don't need to be irrevocable. But the ability to make them so is something that could have a very significant implications—potentially negative.

As a somewhat tongue in cheek example, but with a little bit too much reality to be comfortable, this irrevocability might allow you to "create" a paperclip maximizer DAO (incentivized at the social layer, with humans doing the work).