IMO, browser plugins were the way forward. You can have all the fancy stuff that modern computers can do, like Google Maps or even Quake Live, and at the same time most of the web can stay simple.
Well, the Flash plugin has become less popular because many developers disliked it and decided to bet on the open web instead. IMO that was for shortsighted ideological reasons. As far as I know, browser plugins are the only way to have truly decentralized innovation on the Web. Anyone can make a plugin and offer it to users. If plugins are discouraged, then a handful of browser makers become gatekeepers of all new functionality. It may seem okay now because some browser makers are seen as good guys, like Mozilla and Google, but we all remember the time when the biggest browser maker wasn't seen as a good guy, and that can happen again.
What are your arguments against plugins? I'm curious.