So, it was just as I suspected. No dramatic reason Grant Wilson left. It appears that they are positioning themselves for the obvious demise of (jump-the-shark-we-have-Meatloaf-and-the-Atlanta-Housewives-on) the show.
They own an inn that just happens to be haunted in New Hampshire and Grant can position himself as the happy innkeeper, greeting, telling stories of his exploits as a ghost hunter, and be the ambassador until Jason joins him when the show finishes circling the drain (hear that sucking sound?)
The entire show was a giant infomercial for their inn which I will not name because, well, they got an hour-long special to showcase it.
(Prompting from Grant), "So, inn worker, would you say it's been particularly haunted?" (eager employee to keep her job, blinks) "Yes, oh yes."
They were preparing themselves for the time when the show would be gone and they did some smart things like buy an inn where they could continue to be the ghostly caretakers and have people come to them instead of going to the people on road trips, talking tours, and giant ghost hunts with the public. They can occasionally write a book about the ghosts at the inn and retire on their laurels as THE ghost hunting team in America.
Well done, businessmen.
It's not tragic that Grant left the show. He was filling in the spot they needed to slowly kill off the show and yet assure their future as the owners of a ghostly inn. No doubt, their autumns in New Hampshire with ghosts and pumpkins, Halloween and autumn leaf tours will be quite profitable and ideal. What we are seeing is the calculated segway, utilizing their own ghost team to help them, as they move on to the next step--off air.