Well, it can be but the SCF Extension Store and Extension Warehouse make it fairly easy if they are used to install the extensions in the first place. I suppose that it could be made easier if SketchUp could copy the older versions Plugins folder contents but as it is, that's not a good idea. There are some extensions that install files other places which need to go through the proper installation process and it seems from the troubleshooting we "get" to do that an awful lot of the extensions can't be bothered to keep their extensions up to date. And there are frequently updates to the Ruby scripting version which require extensions be updated to work with the new SketchUp version. If the author of the extension didn't get their script updated and the user hadn't already installed that updated version, the extension wouldn't work. Seems like a lot of things outside of Sketchup's control to make that a good idea.
