> Is the kernel simply too complex to reason about?

Yes, and that's well known.

> Is it an underlying flaw in the security model or language implementation?

Yes, both, and that's well known.

> Should we assume that all software is more compromised than the kernel?

No, some software out there is smaller and have more automatically enforced correctness.

We should expect every large software not created over a simple correctness enforcing stack to have the same number of issues. Linux is just a worst case here, so is your internet browser.