Comcast has announced that it will shutter NBCSN "sometime after the Olympics" and that USA Network would "simulcast remaining programming during the transition to accustom viewers to looking for sports there".
While there is no date set by Comcast, since that time they have given up their NHL rights, meaning their last NHL game will be in a couple of weeks. That leaves NASCAR, Indy Car, and English soccer as its only remaining big programming.
My guess would be that they will just start the soccer (starts in August) on USAN, and shut down NBCSN by October first, when there is so much competing sports on all the other channels. They certainly are not going to keep it going on monster truck reruns and such like just to show a couple of low rated NASCAR races against football.