I really wonder why we need the JVM at all.

For Haskell at least, the GHC runtime is getting pretty good, and seems to have at least basic tools for GC analysis etc. I'm ignorant of MLs so perhaps someone can speak to their status.

Of course there's the much-touted "Java Ecosystem" but it's looking crustier than ever methinks. Maven?? Blech! Also, it doesn't seem wise anymore to build a website on Servlets/JSP/SpringMVC etc, esp. if you want to attract devs. Rest APIs maybe.

What's the killer feature these days recommending the JVM?

Reliability, performance, maturity, instrumentation, diagnostics.

Just picking one example from the list, performance: How is GHC's runtime dealing with 100GB+ heaps?

Do the Oracle or OpenJDK JVMs deal well with heaps that large? I have no interest in other JVMs because my employer would not consider switching and I can't say I blame them.

It depends on what you mean by "deal well with heaps that large". You can certainly set them that large with no problems. But given your memory patterns they can cause dramatically bad GC times.

Yes. It's routinely done.