I mean, I know they said they would support the game for 3 to 4 years, but for all we know that could just imply technical support, bug fixes etc. It's unlikely, but you never know. It doesn't guarantee that you'll still be getting new content in 4 years time.
You seem confident that we will, which is fine (though going as far as to suggest the user you quoted didn't understand what they were talking about was a little weird. They just had a different opinion to you
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) I'm not sure I share your faith. You're not wrong to want or expect more content, but no matter how badly we want it, they don't necessarily
need to keep things fresh for old players. The updates are free (so far, at least) and Nintendo already has your money. Sure, they get a certain amount of cash from people maintaining online subscriptions, but it's not like that's an Animal Crossing exclusive thing. They know new players will continue to buy and enjoy the game for years to come, just as they continue to buy and enjoy New Leaf. Animal Crossing isn't some huge pay-monthly MMO - they don't need to incentivise you to keep playing forever, because they're not getting anything more from you. They only need to do the bare minimum to retain customer faith in the company, and that's it. They'll take our feedback and maybe change things up for the next instalment, and take our money all over again.
Again, I think it's unlikely that they won't continue to add to the game. I think we will continue to get updates, but I certainly don't expect many of the huge changes or massive additions people seem to be so sure are coming. I don't even know whether I think they're needed, but that's just personal opinion. There aren't many games that are intended to be played for years and years; that's why they make new ones, after all.