Interesting, because I would bet that there will be a serious web app contender in the CAD sphere at some point. I'm not so sure the OP is off the mark here.
Maybe someday, but I just can't see a web app that could seriously compete with the likes of Adobe or Autodesk in the foreseeable future. While there are online services that provide some of the features found in applications like photoshop, they are hardly compelling enough to get any Adobe or Autodesk customer to jump ship. Same goes for the like of Siemens and Mastercam, but probably more so.
