The core gameplay of Vampire Survivors was directly inspired by a South Korean mobile game called Magic Survival, developed by an indie creator named LEME and released on Android in late 2019.

The community are playing the main games,

People are playing Vampire Survivors because it's Vampire Survivors. No one can say whether it would be the Flagship of its genre if genAI let someone create a passable knock-off in a week. Yeah, we've always had knock-offs but those required some modicum of effort to produce. And that effort took time, time where the original could become entrenched with people that like what it innovated on.

We can't talk numbers because nobody has them, but I'm pessimistic that the majority of Vampire Survivors (or any game that has reached "mainstream") players are engaging with the game in a way to interfaces with what makes it unique. They would get the same enjoyment out of a shallow rip-off. They're only going with VS or any mainstream because other people have vetted it as "not a waste of money" or out of FOMO. Idk how old you are but I absolutely "enjoyed" N64 shovelware games that were objectively 6 out of 10s because they were good enough. And I have a more discerning palate now, but that's because I'm a fan of the industry. Most people are just consuming content, like watching basic TV. The "fans" you describe are people in these comments. And they are indisputably in the minority when it comes to volume of product sales.