I would argue the benefit is also it’s declarative, done forever, and your machine becomes relatively bulletproof.

Dev environment issues are a thing of the past, once you’ve defined your configuration.

If something is broken with a package, I don’t have to figure it out myself —- I just rollback, wait for someone to fix it upstream in nixpkgs and pull down the patch later.

At least in my opinion, the leverage here isn't about directly saving time in some hypothetical universe where you set up new devices every week.

It's about confidence in your ability to quickly bootstrap a productive system and the relative freedom/security that flow from knowing it.

When you know you can be productive this quickly without access to a backup or a working device, you have relative freedom and security from a decent spectrum of manufacturing defects, hardware failures, disasters, accidents, thieves, and so on.