But the central idea that the server system can only run one program makes a lot of sense to me. Obviously it won't work for a desktop, but then the whole idea that desktops and servers run the same OS doesn't make much sense to me.
Say I have an web API written in Go. For some reason I need to do some openssl stuff before I start up my main program. In a container I can just wrap my program in a little shell script which generates/signs a cert before starting the API.
Such a simple task. It is gonna be a lot harder with a Go Unikernel as I could have to bake this functionality into the application itself.
This Swiss Army knife that is Unix often has it's uses.
In any case, in you have a web API written in Java then you can connect advanced monitoring tools like JDK Flight Recorder and directly affect the system via JMX beans.
So it is more a matter of tooling maturity than anything else.
But the reduction in attack surface of not having all those Swiss Army blades hanging about - and not having to configure them just right so I don't accidentally damage stuff - is a real benefit.
In those cases 99% of the time you really don't care what runs underneath the language runtime.