Earlier quoted context omitted.
Please re-read. Browsers don't have exemplary security, but, what they have can be slightly better than simple code signing. Also, I say it's entirely possible that the other platforms to leapfrog them. The problem with current sandboxes for native code, is that they disarm the participants while empowering malicious code.
I am rereading. I am also asking for specific examples of how a browser has potentially better security than a custom application. Because, I just don't see it. Unless you are trying to say that something in a limited sandbox of a browser has limited capability outside of said sandbox. But... how is that not just as true of any other sandbox approach.
Right now, native code security is pretty much limited to code signing and sandboxing. Browsers go one better than code signing with restriction of communication. Additionally, entire classes of exploits are made much harder because Javascript is a managed language.