I don't think thw committee's options are really so limited. They could totally have defined a JavaScript 2 not backward compatible with JavaScript 1. Browsers

I don't think thw committee's options are really so limited. They could totally have defined a JavaScript 2 not backward compatible with JavaScript 1. Browsers, of course would have then had to support both, but who cares? Especially if there were a good FFI between the two, noone would have been inconvenienced (except browser developers)