Loading standard solution
Much as it might mostly be bad practice to directly edit the standard solution without adding your cusomizations to any specific one this choice (and risk) should probably be given to the user.
Thanks for the suggestion!
The reason that the Ribbon Workbench doesn’t allow loading of the default solution is that it would result in very long download/upload times.
The only way to edit the ribbonxml is by downloading, unpacking, repacking and uploading the solution.
For more information – see https://ribbonworkbench.uservoice.com/knowledgebase/articles/169819-speed-up-publishing-of-solutions