A good rule of thumb, but subject to at least one major hole. Sometimes, the new thing has (effectively) permanent staying power (albeit usually less than the peak of the initial 'fad' phase). Notable examples in our lifetime include Dungeons & Dragons and Magic: The Gathering. They followed the initial spike and drop-off curve, but never dropped *very* low, and ended up being the foundation of permanent new genres of play. PG has a decent shot at doing so, IMO.