I think a big difference with Vanilla, TBC, and even Wrath to a small degree, is that the "catch up" gear wasn't there. The previous raid tiers still mattered, they were still things plenty of people continued to do as alts or more casual players progressed their characters. Out gearing older expansion content had a feeling of "that is something this character doesn't need to do anymore." But you might still help other people out, it was always "current content" for someone.
None of that is the case now. Now a new patch launches, and catch up gear replaces entire raid content without stepping foot in a dungeon. Uldir isn't just old content, it's irrelevant content. For the entire playerbase, over the span of a single patch day. Doesn't matter what level you are or how geared you are, the entire raid might as well not exist anymore.
I think that feeling of "meaninglessness" people describe is due to knowing that current expansion content is going to be a transmog run within a couple months. The value for completing it and farming it doesn't stick, within each expansion, like it used to. Working to gear through/past a raids content seems silly when, in a few months, fresh 120's in greens won't even look twice at it.