>>49508572
I think that the reason why the otaku have slowly disappeared is the normalization and the subsequent degradation of anime and other forms of otaku culture.
In other words otaku culture has become mainstream, which attracted normies who didn't (and still don't) care about the quality of the works they consume, which created this negative feedback loop of normies joining in and studios creating worse and worse content because people would still watch it.
Also the rise of streaming sites didn't help.