I'd say that the separation is not just about security. It's also about performance and stability through separation of duties. The kernelb does a lot of work that is but directly related to current activity in userland. A good portion of this is to keep hardware in a state where userland programs can reliably run without having to reimplement low level functionality.

Additionally, when it comes to performance, it's worth looking at monolith vs microkernels. It may seem counter-intuitive but, for general-purpose computing, monolithic kernels outperform microkernels by a wide margin. This is due to the near exponential increase in context switching that ends up being required by microkernels for the sorts of tasks needed for such use cases.