@MoyaTheGreat I don't think its true. First, seinen doesn't have a specific setting, Kaguya-sama is seinen yet its still a romcom school-related and many shounen doesn't have a school-setting. Second, the school setting is very popular among adult viewers especially for the nostalgic factor. It's always going to sell. The age audience is not really a definitive factor. Third, seinen is not even massively popular among the adult audience, also series with a mosly adult cast like Kaijuu 8-gou are still shounen so it isn't true seinen is getting more popular. Seinen is always going to be less popular than a shounen for obvious reasons. Kaguya-sama is a rare case of a seinen being popular since people watch it for the comedy, romance and even the school setting, not for the seinen aspect. This also applies to other seinen with a school-setting.
You're confusing demographics with the setting and the age audience. Even in the extremely unlikely event seinen is more produced, the audience is not going to like seeing Anime with adults doing boring things. And if they do interesting things, be prepared to read comments saying that they "don't act like an adult" or "don't look like an adult". This is one of the most constant criticisms for Kanojo, Okarishimasu and Kaijuu 8-gou. I even checked the seinen list and several Anime don't have an adult cast/full adult cast, they are school-related or even comedy, at least 3 even don't have adults in the main cast despite being seinen: Medalist, Ao Ashi, Kimi no Koto ga Daidaidaidaidaisuki na 100-nin no Kanojo, Otaku ni Yasashii Gal wa Inai!?, Shiunji-ke no Kodomotachi, Tongari Boushi no Atelier, Snowball Earth, [Oshi no Ko], Kirei ni Shitemoraemasu ka, Wandance, Sono Bisque Doll wa Koi wo Suru.
School-related Anime is not even that disliked as other genres so I don't see how they are going to be less produced if school is always going to sell especially since its a familiar setting for everyone.
Super no Ura de Yani Suu Futari, for example, is a very rare Anime where all characters, both main and side, are adults and its seinen at the same time which it hasn't happened since (from my memory and excluding non-romance seinen like My Home Hero) Wotaku ni Koi wa Muzukashii (not even a seinen). Even so, most people like Wotakou due to the romance, comedy and otaku culture and not due to the "age" of the cast. The most ironic part the most popular series with an adult cast nowdays is Kanojo, Okarishimasu, a shounen.