That said, isn't it the case that one can't just take something that exists in common usage and trademark it as-is?
For example, the basic structure of the fairy tale about Aladdin and the magic lamp had been around for a long time, and was clearly not originated by Disney. Consequently, Disney has no right to claim trademark or copyright control over the name Aladdin or or anything generally related to the story. They only get rights over their own work - additions they made to the story, characters' appearances, etc.
Similarly, Games Workshop shouldn't be able to own the phrase 'space marines'. At most, all they should get to control is the use of the name Space Marines to describe the surgically-enhanced cybernetic super-soldiers that are peculiar to their franchise.