Almost all ES5.1 code will run on a browser that implements ES2015. There are subtle breakages like there were when moving from ES3 to ES5.1, but affected code is rare (and probably bad anyway). If `arguments` were changed, it would break huge swaths of the web rather than a couple sites.
While it would be nice to require everyone to re-write their website code, the cost would probably be in the hundreds of billions of dollars. The pragmatic choice is to leave it in place and discourage it's use. Maybe it can be deprecated in a couple decades.