I think the successor is multikernel. Modern systems trip over themselves to provide isolated runtime environments within one kernel, but it's all a waste once

I think the successor is multikernel. Modern systems trip over themselves to provide isolated runtime environments within one kernel, but it's all a waste once you pop a hole in the one kernel. The answer to that is virtualization of entire kernels, ala Firecracker. Rather than tying ourselves into knots trying to extend the one kernel, we can just run 50 kernels, and have abstractions to connect them into a distributed OS (think Plan9). This allows the kernel to be simpler, provides an easier path to standardized interfaces between components, and lets us infinitely scale a single system, with stronger isolation guarantees. It's basically a microkernel but the components are farther apart. And because of that you can do things like have multiple different kernel versions interoperate over one stable abstraction/API/whatever. SSI clusters are another example of this, they just happen to be using the same kernel, but they don't have to.