I think java apps failed to achieve more mainly because they didn't have decent access to the DOM, and the HTML rendering engine that goes with it.

IMHO that's the killer app of the browser... for all it's faults, the HTML+CSS rendering engine is a superior UI description api than anything previously devised.

I remember the brief period I wrote some java apps. The main pain was choosing between rendering a native java gui, or establishing hooks to have it instrument an html interface from behind the scenes. The native gui had better connection to the code... whereas using the DOM was insanely flexible, but talking to the DOM was like trying to drive a car by reaching through the tailpipe.

That I think is why java apps never prospered (outside of the whole "loading" issue... which could have been optimized away if the motivation was there).

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JS currently has the best of both worlds, and not because of anything native to the language (though it does have many positive points).

I feel like WebAssembly, and things like PyPy.js, are a step in the right direction. Making the DOM and HTML+CSS just another ui library available to the language of the moment. Having PyPy.js is great, because (I assume) where PyPy's python goes, all the other pypy interpreters (Hippy, etc) can follow.