I'm not sure I think the exokernel/unikernel approach by itself is the path forward. While the library operating system approach makes a lot of sense for applications where raw throughput/performance are crucial, they don't offer as much in the way of development luxuries, stability, or security as most modern operating systems. Furthermore, outside of very specific applications the bare metal kind of performance that the exokernel approach promises isn't really that useful. That said, I suspect a hybrid approach may be viable where two extremes are offered, an extremely well isolated and secure microkernel which offers all of the luxuries of a modern operating system built on top of an exokernel which can also be accessed directly through the library operating system approach for specific performance critical applications (say, network and disk operations for a server.)
There's been research into using Linux as the basis for having a unikernel for performance-critical workloads (e.g. database) while retaining all the development /performance-optimization/etc. tooling for both developing the unikernel and for all the other workloads. Of course that doesn't give you a small OS codebase but it does let you optimize for specific workloads.
