I've never accepted the idea that ich is present in all aquaria, just waiting for stressed fish for it to suddenly re-appear. I believe that this is just a rationale made by those who have inadvertently imported it from somewhere else.
There is division among knowledgeable authorities on this certainly. But those who do not accept it are faced with a somewhat insoluble circumstance. If an aquarium has been running one full year or longer, with nothing new in the way of fish, plants, wood, etc that might bring ich with it entering the tank during that year, and the heater fails overnight causing a significant lowering of temperature, ich will inevitable be obvious in a day or two. Where did it come from if it was not already there, waiting for the stress of the fish to break down their defenses?