https://steamdb.info/app/1671210/charts/
The game is estimated to have nearly 1 million owned copies, and that's just on steam, and had over 100K players running the game at its peak.
https://steamdb.info/app/391540/charts/
Undertale has at least 6 million owned copies, with over 200K players running it at its peak.
I wouldn't consider this game to be niche. It does have a fandom, but it's a game enjoyed vastly by more people outside the fandom. I had to convince my friend to play despite avoiding it due to the obsessive fandom (which I have nothing against), and they genuinely enjoyed it, even when a "steam friend" called them the f-slur for playing it. I don't think it's fair or true to say this game is solely made for a fandom to obsess over when the game accrued wide popularity across all kinds of people.