"at least one of three"

> What was Python's killer app?

Not a killer app, but the fact you have so many ML frameworks

> What is Rust's killer app?

That's clearly Servo.

> Go?

It was docker which really made Go take off, it was their first big platform.

> Kotlin's?

Android compatibility, before that it was a niche language not being picked up much.

Python already won before ML was a thing.

In his early days, Python did not need anything, it was simply nicer to use than the other languages. Nowadays ML is a big plus.

It was called Zope.

Killer app is the wrong phrase. Rather every new language needs a competitive upside over existing languages to be successful. At which point your proposed questions have easy answers.

C++ and Javascript are definitely not counter examples as I commented.

Outside of the HN bubble python is the only one of those that got off the ground so far.