The website exists for more than 3 years, today we are the 7th of October and the donations are already at 100% for the whole month. It's arguably sustainable enough.
Traceur (ES6 compiler) itself has a runtime of several hundreds kb. Nothing you can't avoid at some point. There is an overhead for sure. But in most cases, execution time is spent in native calls (Dom rendering especially).
Hm, the reactive manifesto doesn't say that it's new. It tries to convert people from bloated ol' Java/CSharp to better practices that fits new common needs. Of course Reactive is not new... Nor realtime... But an ecommerce website for instance never had to wonder about this kind of things before: new needs.