> essentially reboot into a renewed environment. Is there a standard way
> to do this without an initramfs or a close equivalent?

That isn't quite what an initramfs does. It doesn't reboot into a renewed environment, it gives you early access to all the same binaries and configuration files that were copied from your root filesystem to make the initramfs image.

Your statement of loading CPU microcode as this being a 3rd reason is valid. This is more relevant to SMP systems commonly used as workstations and servers which boot using a single core.

If there is concern with the CPU in use, there are two courses of action, use an initramfs and remake it every time a new microcode update is released.
Or embed the firmware into the kernel image and remake it every time a new microcode update is released.

As a kernel no longer takes 45 minutes to compile ....