>>35801046Gen 3 may have killed Pokemania, but nothing about the games themselves was overly bad. Yeah, the lack of transfers sucked, and it was when they started changing the designs, but I don't think anything would have kept up the hype, since a lot of people just left due to getting older at that point.
I think Generation 4 wasn't very good, but isn't where things truly started to decline. Diamond and Pearl had some questionable design decisions in regards to HMs, pokemon distribution, as well as some hardware issues. On the whole though, they are a progression of earlier games philosophically. There's still some freedom with Gym order, and a focus is put on exploration, even if HMs made it more tedious. Plus the existence of HGSS shows that there was still a commitment to improve.
Gen 5 is where things get shaky. It feels like the first game where they sat down and tried to think of how to shake things up for whatever reason. So you get a much larger emphasis on story, an entirely new pokedex, and less of a focus on exploration. There were also some improvements there, such as seasons and a lot of the more background features that showed they were still putting in the effort, but gen 5 fell flat not because of poor execution like in gen 4, but poor direction. It's where they started with the excess of friends and cutscenes and plot and forcing you to catch legendaries to progress. Even though it had features like Hard mode, the more superficial elements with the pokedex and story gave it a bad impression for most people.
And at that point, Gamefreak probably realized that they didn't need to put in as much effort as they had been, especially since they had poured in effort into gen 5 without any major payoff. So at that point they just decided they would make things look nice, make a basic, mostly linear experience so that younger kids could play it more easily, and not spend a bunch of development time on extra features that weren't appreciated enough
