It's also easy enough to just add a mount command to your initramfs init, or embed required device nodes in the CPIO.
You don't even need a devtmpfs in the initramfs environment, and it may or may not even persist into the fully booted system. I believe Systemd has a tmpfs unit file, and OpenRC has a service for it which is enabled by default.
In this case, my point is for a system where the "fully booted" state is still within an initramfs. There is no "new root" past the initramfs, it IS the rootfs which will run forever until the system is rebooted or shutdown.