I think this number is overblown. Production useful doesn't have to mean 1:1.
Running it without all graphics drivers would be fine for server use. Also, not all filesystems need to be ported: basic ones should be enough for start. But not only servers, home routers run Linux kernel...
If every OEM starts contributing their drivers in rust, this could move quickly...
The main reason is that Dbus is not available during early stages of boot. There are many others.
Varlink seems to be better or the same compared to dbus in all except two things:
Details here:
https://media.ccc.de/v/all-systems-go-2024-276-varlink-now-