Earlier quoted context omitted.
Ok, but that would only be true if I was going to spin up a new vm to join the load group / cluster but even then no one would do that unless behind a reverse proxy / waf load balancer so the attack vector angle is covered. I'm not saying MirageOS doesn't sound neat, I'm just not yet convinced it has a general purpose use case.
Due to the startup time your average server would, in practice, still need to run an SSH server for remote administration which is a vector for attack. A MirageOS server you would instead just tear down and start a new version of. 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…
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.