Well... to be honest, Shonen anime is not the only genre out there, and the thing anime fans or otakus are watching tends to shift from time to time. So, shonen anime won't always be everyone's popular series. Even less now
To me there is as much shonen as there is isekai, too much of the same thing or the same tropes repeated over again at the point of not being that interesting anymore.
Right now, at least from the communities I'm on, a lot of anime fans are prioritizing other genres like fantasy, seinen and such.
Anime, in general, has normalized and expanded. Is not like back on the days where you could only consume what was being translated, dubbed or subbed online or via TV/Local bookstores, therefore why everyone into anime liked and watched the same shows.
Nowadays you have enough to choose and refine your tastes in anime, and since communities didn't conglomerate in a single space due to the lack of content, now they can separate and diversify, therefore why it looks like there are less fans nowadays.