Brian Kernighan on the origins of Unix

I can't help but think that what you're asking for is a micro-kernel-like design from the ground up, where e.g. the user graphical session is its own set of processes (possibly related via a cgroup or namespace-like idiom), each browser tab is its own process and each container is its own proces and where all these processes use the fundamental micro-kernel message-passing paradigm (possibly wrapped) to communicate with each other?

If all processes used the same form of containerisation/namespacing/layering and the same means of IPC (possibly with some faster-than-TCP-but-still-reliable network transport available), this might function as seamlessly as one could ideally expect.

At least, that's the endpoint my imagination extrapolates to from your problem statement. I could be (very) wrong of course.

I don't know how close Plan 9 comes to this hypothetical scenario or whether it's even close. But perhaps Zircon/Fuchsia might very well become the closest modern approximation of what I outline?