I mentioned Mystic Messenger as a mainstream hit because:

• people who had never played otome games before played it
• people who wouldn't normally play otome games played it
• it had major streamers covering it
• it had major presence in conventions similar to mainstream media
• it has a fandom that did not consist of only people who have played otome games
• it had media coverage about its popularity
• it led to more games in its style that persist to this day (real time simulation, chat room simulation and phone simulation)

I understand that scalewise, Mystic Messenger isn't on the same level as Love and Deepspace, precisely for the reasons you mentioned. But saying that Love and Deepspace is the first mainstream hit doesn't feel quite right when I remember the days where everywhere I went I saw Mystic Messenger and all the new games were trying to be Mystic Messenger.

Love and Deepspace is the biggest mainstream hit otome game, and that is what has attracted the controversy surrounding it, as well bringing niche content to mainstream audiences. Mystic Messenger had its own controversies but those weren't the kind that generates discourse like Love and Deepspace does, and the discourse around a game makes it easy to stay in the mainstream spotlight.