There are three main purposes that an operating system fulfills:
Operating systems allocate resources between different applications, deciding when they will receive central processing unit (CPU) time or space in memory.
Operating systems provide an interface that abstracts the details of hardware access
Operating systems provide common services, such as an interface for accessing network and disk devices.
LLMs will can not and will not ever do these things, and your idea that an OS will become a tiny thing like a BIOS is completely wrong.
If these things were true, an OS wouldnt need a BIOS by now, they would have merged into the same thing long ago. But they havent because they are distinct layers which require certain functionality performed before handing off to the next layer. This is why an LLM will never be an OS, and will always be an app that an OS hands off to.