CBS may simply be waiting for the RIGHT case... a good case they feel they can win. The worst thing that could happen to them would be to pick the WRONG fight. Compare the lawsuit DC had with Marvel in the late 1940s-early 1950s to one they had in the late 1990s. The first they won mightily, making the use of ANY cape on a super-hero a copyright violation, almost the most minute aspect of Superman was covered by their 'Franchise copyright.' Fast forward fifty years. They have NEVER stopped producing Superman. There has NEVER been a two month period in which no comic book came out with their iconic character... but the results were very different. The franchise rights the court drew were much narrower.
Franchise copyright is a relatively recent idea in intellectual property law, and it is still forming. The worst thing that could happen for CBS would be if they sued a fan film that used only a small part of their universe, perhaps Star Trek: Aurora (which is NOT crowd funded) and they LOST. The story is a 3D animation, which they have never produced. The characters are original, and are not in Starfleet or any other official capacity for the Federation. Star Trek is the background for an original story set under original circumstances with original characters. A court might find that to be 'fair use.'
Their perfect target would use their characters, their styles and sets made to look like theirs, their themes, pay for the actors, directors, CGI people, and release them in a way that sounds like they are associated with CBS in some way using a hard-sell ad campaign, come-ons that seem to be trying to engage their viewers when they aren't releasing much, but then follow that up with a big fundraising event. If they can't win that one, they no longer own Star Trek in any real sense. I'd venture to say if they can't win that one, they won't be able to stop Disney or any other pro studio from making Star Trek for profit.
CBS's claim to TOS designs and styles (and even TNG-VOY-DS9 designs and styles) is much weaker than DC comics, because Franchise Copyright has two parents, traditional copyright and traditional trademark, and they haven't used the TOS styles since the 1960s. Any claim related to trademark is history. In contrast, if you look at Action No. 1 from the 1930s, you could recognize the costume of today's Superman comic instantly.
The longer they wait, the weaker their claim. I will be surprised if we reach 2017 and no lawsuit has been filed against anyone who ignores a CBS C&D letter. But I do think they are waiting for the right time and the right case. They have to knock this one out of the park to protect their property.