For a certain definition of "large" and for certain domains ( especially finance ) his argument is very, very valid. 200 average developers working with GWT->JS are anyday going to be more productive than coding native JS to get the same functionality. This isn't anti-JS...end of the day if you are on the web your code is JS. Thing is, how do you get there - do you hire 20 pricey smart JQuery guys, or 200 average Jav…
Replacing swing UI's seems like a reasonable thing to tackle using HTML/CSS/JavaScript. You could just pass data back and forth over HTTP if you are talking remote client/server or bundle a local webserver if running host only.
In the case of a platform like Windows 8, you can write actual apps using JavaScript (no server) but you can also write libraries in native code and access them directly from the JS.
I think this is just a classic case of "use the right tool for the job".