You are talking about a single user system.

You can compartmentalise further.

Modern processors even give you these 'rings' to help with the hardware side of that, if you want to.

Yes, these are the standard excuses. But has anyone actually thought about them in the last twenty years, in the context of the way we actually use computers?

Eh, yes? Just look the discussions surrounding the development of any web browser? Or look at the permissions system in Android or iOS or on Mac (and perhaps even Windows?).

So you acknowledge a browser bug is just as bad as a kernel bug.

A bug in the website itself is just as bad, if it's a website you care about.

They differ in how easy a fix is to develop and roll out, and what the blast radius is.