It is not much of an argument though: it is just opinion that Java is the world's best language. If the argument is that this is because it can "do anything", then a counterexample easier than trying to prove that an OS cannot be written in Java is to point out that assembly language, C, C++, etc, can also "do anything". In the end, use the best tool for the job if you can, rather than always try to use the "world's best programming language" even when it (whatever it is) is not the best tool for the job.
One of my teachers at college thinks Java is the worlds best language and can do anything. I dont remember how it came up but someone said that Java can not make an OS. He said otherwise and because no one could specifically say why Java cant do that, he wins the argument.