modern online games are already using web scaling approaches
There really are two kinds of things going on in most games.
One is the real-time component, where many other players see my real-time movements and my real-time actions.
The other is the persistent game state component, which is not that different from any other web services application.
The trick is that, if you try to apply web services approaches to the real-time stuff, you will fail, and if you try to apply the real-time approach to the web services stuff, you will pay way too much to build whatever you want to build.
Some projects have tried to unify these two worlds in one way or another (for example, Project Darkstar) but it's never really worked out well. In my opinion, doing that is a bit like trying to unify real-time voice communication with one-way streamed HD video delivery -- the requirements are so drastically different, it just doesn't make any sense to do that.
That analogy isn't entirely bad: One-way streamed video is a lot more like "web apps" than real-time voice communication, which is more like interactive games, except without the N-squared interactions and physics rules.
So, one way of approaching the scalability problem is to dumb down the gameplay until it's no longer a real-time, interactive, shared world. That doesn't make for fun games for those people who want those things. The other way is to be careful about only building the bits that you really do need for your desired gameplay, and optimizing those parts to the best of the state of the art.
Getting the game developers, and the web developers, to understand each other, and work under the same roof, AND making sure that the right tool is used for the right problem, while delivering the needed infrastructure for gameplay development, is really hard to pull off; I've seen many projects die because it misses one of those "legs" and doesn't even know what it's missing. (A bit of the Dunning-Kruger effect, coupled with "I've got a great hammer!")