I think maybe is the answer it managed to barely do just as much backwards incompat as it could get away with. (Barely, it drove a lot of people crazy). But it was quite possible to write code that worked in both ruby 1.8 and 1.9, quite intentionally, despite the backwards incompatibilities.

Combined with a commitment from key players (esp Rails) to move to 1.9 and stop supporting 1.8, forcing the issue. And that it happened at a point when ruby/rails were at the peak of their popularity, not already on the downslide (Perl). Which of course effected how much backward incompat you can get away with.

That's the trick, predicting how much backwards incompat you can get away with, of course.

But of course, there's another lesson here too, which the OP astutely calls out in the "greenfield" section -- if you focus too much on stability and supporting existing projects, that's just another way to make the ecosystem go stale.

When every major browser is moving in the same direction with a language, any server-side counterpart is best served moving in the same direction.

Without browsers, io.js would have likely ended up in the same place as all the examples above.

It seems too soon to tell what's going to happen with io.js. It was released just a few days ago.

what is considered good c++ has changed pretty dramatically from what I learned in the 90s. Though afaik none of those changes were breaking.