Who knows, Unity3D will substitute it for games, HTML5 Video /Sound for playing videos. While that happens, we will soon realize that Unity3D is another plugin-in-browser solution and might be (and I am just speculating here) vulnerable to issues similar to flash; while content providers will stop pushing video content because of lack of DRM like technology with HTML5 Videos. Please note that I am NOT supporting/rejecting any of the mentioned technologies; this is just a reply to a straw man argument.
But the lack of applets is pretty apparent to me. I come across Flash almost hourly while my last few encounters with Java Applets were academic demos 2 years back.