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It'll never be that popular again.
This isn't necessarily because the games have declined in quality. Phenomena like the early pokemon craze isn't something you can produce artificially. The games just happened to be released at exactly the right time to massively resonate with a generation. It's a cultural thing, and it's nearly impossible to predict by it's very nature. Stuff just clicks sometimes, scratches an itch we didn't know we had.
At this point, while the series is firmly entrenched in gaming culture as a longstanding powerhouse, it's worn off a lot of it's luster. The games are still fun, especially for the actual target audience and not the sometimes bitter older crowd of fans still hanging on, but it's just not the right time for that kind of gameplay anymore. The mass appeal faded, just like any other craze. That it's still so popular is a testament to the franchise's power.
This is why I don't think it's fair when I see people reviewing retro games and say "it hasn't aged well, it was never really that good". You can't judge this stuff by how it feels NOW. Maybe the you living in 2018 thinks it's crap, but it meant something entirely different to the child you used to be, and his opinion is just as valid. Moreso, even, since he experienced the game as it was intended to be played, instead of looking back on it tinted by overblown nostalgia and cynicism.