I've always seen "immutable infrastructure" as another riff on the general theme of Postel's quip, LTI systems, functional programming, "just restart it" approaches all the way down, ... The general admission that systems with memories becomes rapidly much harder to reason about than over time than systems without memories.
Memory is of course also the only way to do anything useful when one interacts with the real world, it's very hard to eke out a decent living being an amnesiac genius at anything. I also think well-designed memory systems become harder to reason about much slower than poorly-designed ones -- take, for example, that NetBSD box that one guy spun up as a simple file server that was still working untouched and forgotten about 10 years later. There's bound to be some weird stuff happening on that system, but even so you could probably remote in and mostly find a system you could navigate and administer.
But when you don't have the luxury of time and experience to design such a graceful system (and none of us with day jobs do) you can reap a lot of those benefits by just trying to be as immutable as possible before you start causing major performance issues.
Memory is of course also the only way to do anything useful when one interacts with the real world, it's very hard to eke out a decent living being an amnesiac genius at anything. I also think well-designed memory systems become harder to reason about much slower than poorly-designed ones -- take, for example, that NetBSD box that one guy spun up as a simple file server that was still working untouched and forgotten about 10 years later. There's bound to be some weird stuff happening on that system, but even so you could probably remote in and mostly find a system you could navigate and administer.
But when you don't have the luxury of time and experience to design such a graceful system (and none of us with day jobs do) you can reap a lot of those benefits by just trying to be as immutable as possible before you start causing major performance issues.