What I am talking here about is this. Most historians are aware of ground level facts like: decline of urbanism, trade, loss of a bunch of literature, political chaos, and Eastern Roman and Arabic world overtaking the former Latin part of the Empire that used to be at least nominally dominating. That's why I precisely indicated my geographic scope by the way. 'Western Europe' does not equal 'humans' by any means.
And on top of that you can have some kind of correction and nuance, where you say well, actually it wasn't completely catastrophic, some institutions survived, there were complex social factors and transitions, there used to be oversimplified narratives by Humanists and Enlightenment and so on. Then you have people who only get the correction and nuance part, because this is more recent and in some ways more interesting, and you get to "destroy" some people on the internet with your fresh facts and logic.
But really all that correction part does not erase the basic fact of the matter that this was a decline by most metrics that we can check. Less sources doesn't mean something like 'dog ate them', it was just a society less capable of creating sources, which says something by itself. And as I mentioned, we still have archeology.