@Weetch I think it is more complex than that. I'd ascribe it to a variety of factors, in more or less decreasing order of importance:
i. Taste for this kind of manga has moved off mainstream, not just because of isekai.
ii. Not only isekai, but novel-derived series in general and especially webtoons have undergone a boom that sucked all the air of the room.
iii. This manga is monthly and we're caught up. In the past, my impression is that readers were more patient, but nowadays with so many groups releasing easy-to-edit/translate manga and webtoons on a daily basis, I think the average reader doesn't really have a stomach to follow series that releases only once a month. So they either don't pay them any attention at all, or wait until they're finished to binge. In other words, indulging faster series has caused people to shorten their attention span.
iv. Violence and mystery have never been all that popular, with few exceptions.
But hey, I'm glad the few readers this series has like it! "We few, we happy few, we band of brothers…"
Chapter 11 is in the works, it will be out in a few weeks.