Deploying either as the standard for JS in web browsers would have “broken the web”
Maybe. I think it would have been the same situation we have now with ES6, ES7, TypeScript, Elm, etc. Anyone writing these has to trans-compile to ES5 anyway.
it wasn’t going to bring Flash programming into the mainstream
Nobody really wanted that, not even me. The article was only about the language features. Flash programming includes many other things such as the Flash API (DisplayObject, etc).
I’d say quite the contrary considering the popularity of TypeScript for example.
Truth is we don’t really know what would have happened.