IMO immutability is a byproduct of adopting Terraform--it doesn't manage day 2 stuff like patching, so the solution in some people's mind is "immutability" instead of bringing in another tool to manage the day 2 stuff.

I personally see immutability as a sign of organizational immaturity around system administration, if you're concerned about drift on production systems you don't have the right config management and RBAC in place. It's never more efficient to replace a fleet vs patching it in place, especially from a time standpoint.