All rights are fictitious. All laws are made up. They're simply defined to provide a guide for liberty and order. They're not always enforced. People's defined rights are different over countries and years. Slaves had their rights defined by others throughout history, and many groups considered slavery justified where the master had rights to the slave and the slave had rights defined by the master. Saying copyright itself is fictitious doesn't mean anything.
Copyright rules make sense in protecting inventors and product owners. Maybe you've been neither and can't understand how the lack of this protection harms inventors and owners to the benefit of plagiarists. It's much easier to plagiarize than invent or produce, so without copyright protection, inventors and product owners basically become giant fat cash cows ready to plunder by passers-by that can just copy and paste. Removing copyright would turn the market completely feudalist and chaotic.
Throwing copyright statutes away is also chaotic. Reverting to common law? I'm not sure what you mean there. Just however people feel at the time? We've already evolved past the caveman era.