Isn't it also just because it's old and people get bored of it? People crave new things, and even if it's just as good as in the beginning, it'll get lower ratings because it's not new anymore.

I remember quite some years ago i was like "i'm finally going to watch southpark". And people were already complaining about how the latest seasons were worse than the first seasons. Watched a ton of seasons in a short period, and honestly can't say the later seasons felt any worse than the first ones when you're not bored of the series yet. Now so many years later when i watch some more southpark, it's not as fun as when i started watching it since the "it's new and exciting" feeling is long gone.