It's also not just about monolithic Vs microkernel: Linux could decouple filesystem drivers without becoming a unikernel e.g. by specifying a fixed driver API - you could just load FSs as kernel modules. The reason for tight coupling is just that devs don't want to commit to a stable API. If you are only talking about a single kernel version, FS drivers actually _are_ decoupled this way (can be loaded as modules) and distributors do this.