Intriguing perspective! I respect your skepticism towards the "optimum tire diameter" concept, as it indeed varies based on rider preference and conditions. However, I'd argue that the trend towards wider tires isn't merely a fad, but a response to evolving cycling styles and terrains. For instance, gravel riding, which I'm quite fond of, benefits greatly from wider tires due to their enhanced stability and grip on loose surfaces.
While it's true that wider tires might increase rolling resistance, the comfort and control they offer, especially on rough terrains, can outweigh this drawback. Moreover, advancements in tire technology are continually improving the balance between rolling resistance and traction.
As for aerodynamics, it's a valid concern, but I believe it's more relevant for road cycling or time trials, where marginal gains can make a significant difference. In the broader world of cycling, the joy of exploration and the thrill of conquering diverse terrains often outweigh the need for absolute aerodynamic efficiency.
So, while "optimum tire diameter" might be a myth, the benefits of wider tires, especially in certain cycling disciplines, are quite real.