If a company doesn't have irreplaceable people, then that company is not doing anything interesting. Conversely, if replaceable people can produce your (software) product, then any other company can also do it.
As much as I don't like LLMs that much personally, do you think ChatGPT was produced with replaceable people?
> Companies need to build systems where everyone is replaceable...
No they don't. They need to build systems where everyone is happy with their job and don't need to constantly hop jobs for better salary, environment, etc.
The way to mitigate the bus factor is not to make everybody replaceable; it is to have a process to develop more irreplaceable people with overlapping expertise in their areas.