Interesting. Of course the reference to "developers proposed that copyrighting their work was the only way to prevent what has happened" is a bit misleading. Copyright comes into existence automatically once the game is created, the only question after that is whether the new game breaches the copyright in the old and whether the copyright owner wants to pursue the breach. Whether a breach has occurred can be difficult to ascertain on occasion. This case is a good example of the issues, and it has screen shots of the two games at issue:
http://www.bailii.org/ew/cases/EWHC/Ch/2006/24.html
Also, you need to have the squids to pursue an action, legal protection does not come cheap. Gaming companies could also seek to register design rights for their games, which potentially could give them greater assistance in preventing someone piggy-backing on their work.