There were trends in the past like the magical girl trend, giant robot trend, space opera trend, and high fantasy trend, but those always pulled from things like Japanese women's rejection of staid maturity, Japan's manufacturing boom and the rise of complex electronics, Star Wars and the exocolonization hype of the late 1970s, and Dungeons & Dragons and Wizardry. There was always something new outside of the sphere that got integrated into it. Nowadays many manga are copies of copies, where the things they're copying pulled from games like Tera Online, Xenoblade Chronicles, or Dragon Quest, and from landmark '90s manga and anime like Yu Yu Hakusho, Record Of Lodoss War, and Tenchi Muyo. Especially in the last seven or so years these properties are so many layers removed because of the copying that they don't know why certain plot tropes or character archetypes exist and thus don't know how to use them (or more importantly subvert them).
Sturgeon's Law obviously applies, but currently many of the best examples of ongoing manga and anime lie within a fraction of the diversity that used to exist. Comparing 2024-2026 to 2004-2006 (which was considered a dark age in anime and manga at the time) provides a pretty stark example. The same thing happened with American comics in the 1990s and 2000s before the indie comics boom following Diamond Distribution's years long fight with the industry. While kids don't have a reference for what's good or not because they don't have a back catalogue of experience to compare to, adults absolutely do. And adults are the ones that drive a majority of sales considering the anime boom started in the mid 1990s. They won't read or watch something that repeats a well worn premise but worse. So we might be about to see a collapse of manga as the snake reaches the end of eating it's own tail.