By what measure has GWT failed? It compares favorably with usage numbers to most popular JS frameworks. 130,000 monthly active developers, used on 20,000+ domains, outside of Google, it's used by many large companies, for example, Apple's iAds uses GWT, AWS console uses GWT, major financial institutions and banks are using it.

It has it's niche just like every other framework. Google is now using it to share code between Web, Android, iOS, and Server (shared client side business logic). Google Sheets is the newest example of such Hybrid apps, and the gains are substantial, 60-70% code sharing between platforms, only the UI needs to be reimplemented natively.

Just because something is not monopolizing a particular area of development doesn't mean it's "failed". The ecosystem is large number for many different frameworks to remain vibrant.

To be fair, F# vs. Java is more than a trivial difference.