We agree that a properly sandboxable capability-capable (ugh, lol) language would indeed be a really good security upgrade. I was sad when the SecurityManager died for that reason, even though the reasoning was very understandable.
But those claims have also got to be moderated. As no such thing has ever existed, we can't truly know how well it'd work in practice. Only experience can tell us that.
Global state is one of the key issues. Joe-E simply banned it, which is far too harsh and breaks almost everything. Mobile operating systems locked down filesystem access behind permissions and capabilities quite dramatically and were much more secure, but that came with a lot of 'vigorous' debate over owner control and power for productivity/pro-grade applications. macOS has taken an incremental approach and sandboxes off parts of the FS from apps whilst retaining what looks on the surface like a classical global shared state $HOME and / directory (although it's not).
macOS, iOS, Android and Windows have all been steadily moving code out of the kernel over the years. Apple doesn't run the core FS in a userspace process but every other FS that's not as performance sensitive is now a userspace daemon, for instance. They developed their own FUSE equivalent to do this. In Windows a lot moved out in Vista. Graphics, audio, printing, a lot of drivers are out of kernel now.
Linux has lagged behind quite badly in this respect partly because a microkernel design requires close cooperation between userspace and kernel space but the Linux design philosophy is that the kernel is a self-contained artifact.