Actually those companies only handle slicing a PSD into HTML/CSS -- this is a very small portion of a "web design" job. There's still spec'ing and estimating and implementing it in a CMS and adding custom functionality and testing it and deploying it, and of course actually designing the thing in the first place :)
Agreed. There's also the fundamental problem that directly sliced PSDs create a terrible experience for the user; you can't build smart, responsive web pages that way.
PSDs are good for specs, but not directly converting into the actual end project.
Agreed. Companies like http://www.cssilize.com/ are difficult to compete. They charge as low as $39 and have instant PMS.
On the other hand, there will be huge demand for mobile app themes.