I don't think we actually disagree. Backend language choices have always shown more diversity, and will continue to do so. But still, the number of lines of code and developer hours dedicated to these applications is dominated by the front end. With very few exceptions, UI apps have always been "heavier" than the data engines behind them.
So even though C# won't be going away any time soon, it still shows well below Javascript in terms of "acceptance".
What is changing now is that people / employees have realized that it's convenient to be able to use any browser, from any device (desktop / laptop / tablet / smartphone), from any OS (iOS, Linux / Android, OS X, Windows) on the client-side.
And the one indisputed king inside the browser to code the client-side is JavaScript.
IMHO It's not exactly as if monothreaded JavaScript engines are going to displace server-side Java / C# anytime soon.
Webapps are accessed from clients using JavaScript. But Webapps themselves, at least for many "enterprisey" stuff are very often coded in Java or C# (or at least virtually always in languages targetting either the JVM or .Net).