Within the progression, each new generation is decreasingly tied to a proprietary, revenue-generating product, and increasingly intended for outside consumption. So GMail was completely proprietary and closed off from the outside world. The JS library, compiler, and optimizer from GMail was spun off into Closure, which was widely used across Google products and finally open-sourced 5ish years later, but never got widespread external use because it's very tightly tied to the style of JS development within Google. The idea of JS transpilation was proven by Closure, and Traceur was developed from the outset to be open-sourced and not tied to Google products - but it was developed by a member of the Chrome team, and so the developer perspective was that of how internal Google engineers think, and the development process wasn't really inclusive of outside engineers. Finally, Babel was developed entirely outside Google, based on ideas that had been proven out and were widely disseminated, but it was developed from the start with an open community process.
