I can see immutable infrastructure and this concept working in a copy-on-write model. For instance, if a service is up and running, a modification can consist of duplicating the infrastructure so you have your own copy and then making the modifications. It localizes the change, and in theory keeps the original unmutated.

I have no idea how this would work in practice. This manifesto seems to go against the grain in other ways, as I'm not sure how it can coexist the economic model of SaaS that has emerged. Also, as others have said, the security and UX implications are big concerns that threaten the feasibility of this idea.