I, for one, am happy to see Shanny gone. That guy used play action way too much. The point of play action is to get the secondary to bite. But when every pass is play action, the secondary will stop biting... completely defeating the purpose of the play action.
With the heavy push up the middle due to Mack's injury, all the play action did was give our QB less time to react to the pass rush... because by the end of the season, no team respected it.
As far as the speculation of why he left, well... it's all speculation. Who knows for sure what happened. Maybe Pettine and Farmer didn't like Shanny's lack of offensive adjustments during the course of the season. That seems more plausible to me than the Manziel story line.
But Manziel still sells papers... for now, so of course that horse will be beaten to death.