Microsoft's main argument is that by upgrading the language, rather than creating a new one, old scripts could break and backwards compatibility will be lost. While this could be true, one of the main goals of the ES4 Working Group is to maintain backwards compatibility. This means that, barring a few edge cases, and if all goes as planned, the barrier to entry for ES4 will be minimal, potentially only requiring a browser update (only for previously unsupported features) and, if the developer feels the need for it, training on the new features. However, uptake of a new language will be slow, both on the consumer end where users will have to upgrade their browser or download new plug-ins (anyone want another active-x situation?) and on the developer end where they will need to learn a new language, potentially buy new development tools, and still need to support the users who don't have this new software.
There is also the question of how open Microsoft wants to make it's new language. If history is any indicator, they may try to use it as a wedge to push people back to their browser. Mozilla, however, is an open source company; ECMAScript is an open standard.