The problem is it takes more than just implementing the "kernel" (as in low-level access aka HAL) work under L4. You need to apply that compartmentalization all

The problem is it takes more than just implementing the "kernel" (as in low-level access aka HAL) work under L4. You need to apply that compartmentalization all the way through the stack, and even subdivide applications into smaller chunks of responsibility. What you typically get is a neat subdivision of all the HAL bits, but everyone stops once they're plugging in applications. This is partly because there's normally a large amount of shared/legacy code which needs importing, and partly because people don't recognize the benefits of partitioning an app. It's not all bad - at least you can be reasonably confident that one compromised app, or part of the HAL, can't be trivially used to compromise the rest of the system. But, as you say elsewhere here, if there's just one thing you're doing, then all that effort didn't really improve things.