The point of about transpiling is that it's perfectly possible to build languages with far more pleasant syntax and (for many people) more tolerable paradigms (like class-based inheritance) on top of JS, and the performance characteristics of JS in Chrome and Firefox (as the two platforms now are, pretty much, or their underlying JS engines if you prefer) feel fairly known... so why not do so?
Biggest issue that we've not really seen anything (other than boring bits like type-systems) gain huge amounts of traction.
Coffeescript, a bit, in the Ruby on Rails world for a bit, which made some sense.