Well, the technical sense ("to boot some hardware and coordinate some processes") is too low a barrier.
It's all the other stuff, managing memory efficiently, making the most of software, being fast to make it worthy to run it, making the battery last, etc, being a good server, etc, that make an OS an OS and not a research vehicle/toy/expensive niche workstation (like the LISP machines).
No it obviously isn't. If it boots and coordinates processes, it's an OS.
> expensive niche workstation
In the Lisp machine era, anything that actually had a "real OS" was an expensive workstation (or mini, mainframe, etc).
Affordable consumer machines had software most examples of which didn't control processes, yet was still called some kind of OS: MS-DOS, Mac OS, and the like.
You mean expand it beyond "currently mainstream OS with a big installed base" rather than in any technical sense.
> While OSes have been written in Lisp, no mainstream OS people actually use has. Experimental research OSes don't really count. People have written such things in all kinds of languages, even Java.
Lisp machines were shipping production hardware.
Who has shipped hardware programmed in nothing but Java? Java code taking interrupts, programming DMA controllers, managing memory, etc.