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I think it's going to be when the next generation gets a bit older.
A part if Kanto pandering is 1) It's been around the longest. It's obviously going to have more opportunities to be remembered and mentioned because it's the inception of the series. They also didn't 100% know what they were going to do with the series in the beginning. They didn't plan on Gen 2 in the beginning, and then it was going to be the end of the series. They tied Kanto into Johto to neatly wrap it up, and then afterwards moved to being the bigger franchise that they are. In other words, it was a part of them figuring out their "brand" their series. There's been mentions of every gen in every other gen moving forward, but we've had eight generations where Kanto existed and only four where gen 5 has.
2) Nostalgia sells. A whole generation that started on Kanto that grew up and left the series behind still know Kanto. It helps them get sales, draws in old fans and brings in revenue and customers. As we get older though, I think the generation after us, who started on gen 4 and what not, might start a new wave of nostalgia. The kids who's first game was X/Y or BW. Believe it or not, they aren't sick of Kanto yet because it's relatively unknown to them.
3) It's making them money, at least it was. They are a business, and in the business of making money. Those who complained about so much Kanto were honestly the minority. This is actually the first time when a larger group of people have finally started to voice a tiredness of kanto. It will be interesting to see imehat it means for the future.