It'll be a warmup for the 50th anniversary movie.
You know, that's going to be a mistake; their big misstep.
The 50th anniversary is a very big deal - to a dwindling number of people, most of whom are no longer their target demographic or audience. It's definitely a promotable event to the wider public, but I really doubt that it's worth enough dollars to make pushing a movie out by a certain date a goal that overrides all else.
If after all that these two movies are accomplishing for them the studio execs think that the anniversary is more important than having Abrams's full attention on Star Trek
for a couple of years I think they're probably wrong.