There's "does worse than fat local client" (but better for development and hence a worse-is-better situation) and then there's can't do it at all and will always lag behind so much that it doesn't make sense to think about it.

Are you going to put up servers so that I can solve large-scale quadratic programming and machine vision problems? Are you going to ask me to run a non-linear optimization routine written in JavaScript?!

You expect me to accept circa-1994 graphics in games? You expect me to pay for that?

What kind of dreamworld are you living in?

Google Spreadsheets hemmorages, huffs and puffs if you have more than several dozen rows - it robs power users of most of their keyboard shortcuts and has no kind of extensibility at all. It uses up dozens of MB of RAM with a blank sheet. It's a real Excel killer alright. Which is amazing because Excel has only seen minor, incremental improvements in the last decade.

You're telling me quasi-3D games I played in 1994 on a machine 300x slower than the one I'm using right now constitutes approaching parity with local native code?

In several years, a language like JavaScript will run on the machines available then the way C++ runs on today's machines. But when that happens, machines with many cores will be ubiquitous, and the ridiculous lag will still be present - would you like to run your embarassingly parallel program with 1 core or 32? (1 of course, it's online! Wowzers!)