The elephant in the room being?
The elephant in the room is what I just wrote. A referendum to vote to commit a crime is a crime. And secession is not a walk in the park. Spanish government passivity might have given that impression, but at some moment, it seems to be now, things will start to get very serious.
The follow up argument is the government has the power to amend the constitution but it currently has no interest in doing so. If Catalans want to secede they must first convince their (national) politicians to amend the consitution to make secession legal and then hold a referendum on secession itself.