I'm not saying MirageOS doesn't sound neat, I'm just not yet convinced it has a general purpose use case.
A WAF also still passes through HTTP requests which hit Apache/nginx which calls out into the OS and altogether that give a significant surface area for vulnerability (think e.g. shellshock and ENV variables.)
I also don't think anybody says that MirageOS is ready for the general purpose use case; it's very much specific experimental tooling for (currently) really niche cases.
Unikernels do have a ton of future potential though ....
However, I strongly advise you to read up on how production environments are done in security conscious enterprises (banks, payment providers, etc) as you seem to make some assumptions in your comment. To give you some idea of the environments I've designed in the past:
A WAF worth it's name won't pass any request back to a webserver if it matches a known signature, method, payload, etc. This is a functionality commonly called virtual patching. SSH is usually only allowed on internal vlans and often requires some sort of external authentication mechanism like a centralised jumpserver or ldap.
Also, if using Solaris with OVM for SPARC you wouldn't boot up a new server but a non-global zone (think Docker but 2 years from now) which can be started in ~1 second and offer full isolation from the host system. AIX also does something similar and I've seen several different approaches used on Linux from LXC to VMware + F5 irule based auto-scaling groups.
I guess this is defining things from a purely pragmative "more code means a bigger attack surface" perspective. I know that's an oversimplification but there's also some truth to it.
But indeed; the Solaris OVM / LXC stuff spawning minimal OS's without administrative access gets you quite close (and with a more vetted codebase,) so in that regards unikernels are indeed still mainly an academic exercise.
Did this clarify my point or in any other way contribute to your undestanding?
The overview page and the ASPLOS paper (linked upthread) and the ACM article [1] will help explain the benefits and trade-offs of the unikernel approach. Other people have also started using unikernels so you can read about their experiences too [2].