I've been involved with the 3D-browser market since before VRML. Does anyone remember the Superscape VRT, Visualizer plugin, and the Virtual World Wide Web? The VWWW was a linked 3D world, spanning multiple websites - you could walk from one site to another in a virtual world, back in 1996 or thereabouts.
There's a problem with the economics of 3D content, but in my view it is beginning to shift.
On the one hand, creating 3D content is hard. There's a lot more effort in creating a model of a toaster for an online catalog, as opposed to taking a photo (even a nicely lit, airbrushed, professionally produced one). Time means money.
On the other hand, there is your market. VWWW required a rather hefty download - about an hour on a 28.8kbps modem - and a separate installation process. This limited the number of people who could see the content.
The tools for creating 3D content are also getting more available, and more automatic, and more pre-built models are available than in the VRML days.
WebGL has the possibility to crack the audience side of the equation. Three of the big four browser manufacturers are behind it. It just needs a very successful browser game to force the hand of the fourth. What if the next Minecraft was a browser game?
3D will never replace 2D. But it will become a useful tool alongside it, just as video and Flash do today. WebGL will be an important part of that process.