Maybe when that piece of technology involves taking large amounts of code running at elevated privilege, and that was written by people who assumed that it would NEVER be exposed to untrusted code, and exposing it to untrusted code?
So market pressure will force everybody to do WebGL.
IE has had plenty of critical security holes in the past without WebGL, and avoiding WebGL is not going to magically make them secure in the future. (Just look at the history of the pwn2own contest, for example.)
Secondly, Flash (and I believe Silverlight) will also be allowing users to provide shaders that flash will compile to native languages for GPUs. Which WebGL implementation currently out there do also, they compile the shaders first, check them and test them. (Check out the Angle project). While none of these things eliminate the risks, they are steps in securing the technology and more work is being done to make WebGL as safe as it can be.
The fear that shaders provided by the server can lock up your gpu en in effect crash your computer are also something that is being addressed. There was an article by Gregg Tavares that explained how on Windows, it's possible to time the operation on the GPU and reset the GPU if it takes too long to respond.
As far as running code at elevated privileges. I don't see how that's true. First of all, the browser is running this code in a process that has no access to the file system, other process etc. Drivers are mostly user space with a little bit of kernel space code. I'm sure the risks do exists that that level but again, securing this level should already be something Microsoft be working on and forcing Nvidia/ATI/Intel to do the same. And this isn't exactly an easy hack to do regardless, you have to code stuff for a wide range of drivers to make your hacks effective and also manage to get a lot of people to visit your site.
The worse case here is Intel graphics that use the CPU for some of the work. In this case a bug could be used to execute abitrary code and hack the machine.
I hope I don't sound like I'm saying that the security issues don't exists. What I'm trying to say is while they are there, there is work to fix them and render WebGL safe. WebGL just came out, if in 2-3 years it turns into the hell that ActiveX was then I'll agree it was a bad idea but so far I don't see this getting anywhere close to that level.