The previous behavior was to create the "default" kb namespace when the Servlet started (this is context parameter in the web.xml I believe) and select that name space. The gist of the discussion was what the default values should be for those properties. At the time, the decision was made to change the workbench to not create a KB and not select one be default.
There are clearly downsides. I think your logic would make sense and re-enable creating the default namespace. I have created a sub-task to provide a reasonable error / Exception handling to cover the case where a property graph client tries to connect to a namespace that is not configured for property graphs.