First, there is no reason other than choice for Theodosius to call off the Valentinianic consensus of not persecuting privately practiced paganism. This was not determined by anything. There is no reason he couldn't simply say otherwise.
Second, unfortunately that does not mean it won't eventually happen. It won't mean the death of paganism, the next day (IOTL rural pagan practices seem to have continued into the eighth century on papal domains), but with Christianity dominant, the exclusivist imperative will be there all the time. It just takes one emperor to implement it. There won't be any going back.