Being to aggressive in breaking stuff gets you a shitshow like Node.js or Ruby. Long-term source code compatibility is a very useful feature for open source and a sign of a mature eco system. Feel free to add stuff, but once it's part of a stable release it has to be maintained long after a "better" way to do it comes along.
nodejs itself doesn't have very many breakages; i have plenty of code that is unchanged from 0.12 to 24. npm is a whole other kettle of fish but I don't think you can blame the core project for the sins of everyone that publishes to the package manager. Python2 -> Python3 on the other hand had a lot of breakage in "standard" code.
Javascript would heavily benefit from breaking changes. The reason why it still sucks ass to use today is because this won't ever happen.
I can't speak for node.js specifically but who gives a shit
> Long-term source code compatibility is a very useful feature for open source
Sure, until you need affordable maintainers. Maintainability must be balanced with patience for bad software. Cf the insane maintenance cost of perl scripts