True. But in return we would have had to endure 10 years of in-browser Java.

We would have just built something else on the JVM. You could be writing Ruby in the browser with JRuby, or Python in the browser with Jython, or Scala. This would be a much better world (tooling and performance-wise) than the current compile to high level JS strategy.

As opposed to 10 years of stagnant JavaScript, and a conversion en-masse of all all video, audio, game content to Flash??

It was really sad to see everybody moving to Flash JavaScript could've done video/audio and games a long time ago.

Not necessarily - in that case, we might still be stuck with the Sun-versus-MS and Sun-versus-the-world games over control of the JVM, and with the JVM's startup-time problem. Though the startup-time problem was so bad that it's hard to imagine in-browser Java winning with the problem still there.