We need both - some kinds of applications need direct hardware access. But the kernel attack surface is huge, even with seccomp and friends.
An app on my smartphone or - much worse - an Electron app "sandboxed" in a flatpak on my desktop has access to far wider range of dangerous APIs than a web application. What's wrong with a browser as a high-level OS?
Some of this is aesthetic so I don't really expect to change minds, but if we lived in the world of "Life and Death of Javascript" and booted to some kind of Web OS I'd be annoyed at the loss of low level hackibility and get over it.
Booting to Linux, then booting a browser to get to a normal app that doesn't need network connectivity "feels" wrong.