IE11 (Win8) and various devices that won't receive an update like many Android 4 and older iOS devices will be around for a few years!
Sure, you can use a transpiler and serve JS5 the next ten years (2025). What do you plant to do? It seems most simply haven't thought about the problems lying ahead.
Many were against the ES4/E4X changes that e.g. Firefox supported for some time, as they would have broken backwards compatibility. ES6 breaks backwards compatibility with the new syntax keywords and there is no fallback afaik. If there is a fallback, well then everything would be fine.
Wouldn't it be better if such vendors would ship an security & ES6 syntax support update to their old browser software?
No, people have been against breaking compatibility with old source, like changing how certain statements are parsed and executed that would have made old code now in error. This is why strict mode is opt-in and larger ES4-like changes are dead in the water without careful design.
On the other side of things, though, new features that break in old browsers are added all the time. Try and run Date.now or Function.bind in old browsers and they'll immediately throw an error. It's only through polyfilling (and in some cases transpiling) that sites work in even IE8 today, and it will be the same situation in the future if a site wants to still work on an old Android or iOS device.
But yes, staying up to date as a user does depend on a newish browser being available for the platform you're running, and it would be great if old devices could continue to get updates. Really old iOS is out of luck, but Firefox supports back to Android 2.3, so users stuck there can download that browser to continue browsing the web.
Sure, you can use a transpiler and serve JS5 the next ten years (2025). What do you plant to do? It seems most simply haven't thought about the problems lying ahead.
Many were against the ES4/E4X changes that e.g. Firefox supported for some time, as they would have broken backwards compatibility. ES6 breaks backwards compatibility with the new syntax keywords and there is no fallback afaik. If there is a fallback, well then everything would be fine.
Wouldn't it be better if such vendors would ship an security & ES6 syntax support update to their old browser software?