in a sense, Javascript is a platform, being the defacto standard language for web programming.

It is however a standard in both the de facto and de jure sense - codified by a standards body (ECMA) and widely and interoperably implemented across essentially all modern browsers. In the medium term I would discount a newer scripting language (better or otherwise) becoming the de facto web programming language - standardization has become a given, both legally, and in terms of market expectation, and the cost of implementing a new scripting language, both to implementers and adopters is so significant, that I find it difficult in the medium term to imagine the drivers for the success of a new scripting language.