So, you'd like to expose the whole kernel to Spectre-type attacks. Wonderful!
So the trusted core part of the OS can run without any spectre prevention, though you can still enable the various hardware protections available in the chicken bits.
And if it's necessary to protect against spectre attacks, you can use shim layers or even isolation into ring3 to take preventative measures. This allows leveraging performance were important and security where necessary.
If it's in webassembly, you can even run two versions of a driver; one with spectre-mitigations compiled in and one without, sharing one memory space and the kernel can choose to invoke either one depending on the call chain.
As far as I remember they weren't able to defend from side channel attacks within the same process completely and decided to rely on process isolation instead, estimating it would be too much work to address all known spectre class vulnerabilities on their existing compilers and too hard to ensure for defenses not to be broken later by compiler developers.