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> A stack based machine? Why?

The JVM being a stack-machine is probably the least controversial thing about it. Wasm, CPython and Emacs all also have a stack-based bytecode language. The value, of course, comes from having a generic machine that you can then compile down into whatever machine code you want. Having a register machine doesn't seem very useful, as it's completely unnecessary for the front-end compiler to minimize register usage (the backend compiler will do that for you).

Specifying classpath isn't fun, I agree with that. Launch performance isn't good, and is generally a consequence of its high degree of dynamicism and JIT compiler, though of course there are ways around that (Leyden).

> I've written entire programs in JVM bytecode, without a compiler, and I see very little of value in it

I agree, I also see very little value in manually writing JVM bytecode programs. However, compiling into the JVM classfile format? Pretty darn useful.

> Having a register machine doesn't seem very useful...

Requires fewer instructions, so potentially faster evaluation, which is good for short-lived programs that ends before the JIT kicks in.

Stack machines requires less space per instruction, however, which reduces the size of the program (faster to load).

> Java may be used a lot, but so is Windows. It's an accident of history, of early adoption and network effects, rather than being inherently good technology.

Going on a tangent: Windows is an interesting example to bring up, because the Windows versions everyone uses today have about as much to do with the 'accident of history / early adoption' versions that were based on DOS as using Wine on Linux has.

It would perhaps be like today's JVM being register based, when the first version were stack based.

I don't actually know how much the JVM has changed over time.

> I remember a few decades ago somebody saying the JVM was incredible technology, and as a user and programmer I still have zero clue what the hell they could have been thinking was good about the JVM.

I see what you mean. In that case we can add Scala backed systems as well to the JVM balance sheet. If we simply look at the JVM and the systems it backs, there's very little evidence that it isnt a marvel of technology. It powers more impactful systems than few other technologies.