I can't see how a new film could be expected to make upwards of $100 million more based on a couple of relatively minor factors.
Perhaps not in tickets but ...
they'll get a bunch of free publicity from newspaper articles pointing out "50 years of Star Trek marked with a new film yadda yadda..."
they can profit from other companies using this anniversary to flog their products, and advertise ST at the same time as well, "Coca Cola celebrates 50 years of Star Trek"
also it's easier to sell the back-catalogue if you have a new shiny thing in the display that everyone is talking about, both syndication rights and box sets are more likely to fly out of the shops.
And I'm sure corporate minds will find other ways of profiting from it as well...
2016 is pretty much a lock for the next one.