If you need performance, and portability, and something that can be more easily maintained, for example QT is the only way. Power tools that are graphics heavy, or music applications, or anything that requires heavy multimedia lifting, you're not gonna do that currently in the browser.

Maybe this will change in the upcoming years though also, I have a feeling the client/server bridge will be getting closer and closer as we reach more performant ways of loading code from the servers but executing natively on the client/computer/handheld/phone whatever.

Also, WebGL compatibility is still far away from really doing any production apps (I mean, to reach a state where you can guarantee your app to work on at least most computers people have). Many platforms/GPUs/driver combinations just point out blank refuse to work, or the browser has blacklisted problematic combinations, as these can in reality cause crashing of the whole OS and computer, this has happened to me many times while developing WebGL applications.