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Now that I think about it, the poor performance of Marine's new song was probably a calculated risk. She knew from the start that it won't do as well as her other songs, at least initially. Let's be fair here, almost all of her fans are otakus, and that kind of songs were never popular among otakus and weebs. Kpop-ish musics has always been popular among mainstream music listeners especially zoomers, and that's exactly why she releases that kind of songs: she wants to grow her subscribers by tapping outside of the typical otaku market with their usual Jpop.
Maybe that's also the reason Kobo was chosen. Kobo is an entertainment genius like Marine (at least in the context of vtubing), she has that charm to make anything go viral. Kobo is the biggest vtuber on Tiktok (social media that JP zoomers use the most) and if her fans could help make Marine's new song go viral there, she will get more exposure and in the end a flood of new subscribers.