"Difficulty attempts to keep the block rate to 1 block every 600 seconds. A retarget (when a new network difficulty occurs) happens every 2016 blocks."

Is this in perpetuity. Is that written in stone? Is there any leeway written into the protocol for instances of unpredictability? Just wondering, because I have also read that they actually have a date +/- when the last block will be solved. That to me indicates a finite solution. What happens then?