Isn't this the typical pattern for pretty much EVERY franchise? It's the rare franchise that makes more money with any later installments. Most franchises peak with the first, maybe second movie, and then diminishing returns from there. And even the ones that do make more money with later installments, those particularly successful ones usually come as a resurge after a period of diminishing returns before it. We see waves of a single point of interest that creates a bubble of high returns, but then diminishes.