Earlier quoted context omitted.
In the absence of copyright, one supposes that authors would get large up-front fees for publishing, and that the fee for book 4, book 5 and so on for JK Rowling would have been quite large, so I suppose she would have done just fine. Think "Kickstarter, but for authors".
The exact opposite would happen. No publisher would pay very much for rights to a book that they cannot defend. Once that book hits the shelves, everyone with a printing press will be copying it and selling it at cost, which will undercut the first publisher because they need to include royalties in the price. They certainly aren't going to print a million copies if they aren't sure that they can sell them before som…
In the days before the Berne convention, US publishers copied British books without paying the authors, which of course made the British authors unhappy. In some cases, because there was a bit of a first mover advantage, publishers would send their agents to London to acquire copies of the plates for popular authors as quickly as possible, and were willing to pay a fee for this.