Every time I see someone say something like this I wonder if they actually read the early non-core 5e books, like Hoard of the Dragon Queen, Rise of Tiamat, Princes of the Apocalypse, and Sword Coast Adventurer’s Guide.
I would say any of those books are just as flawed as anything to come out post-Tasha’s. Pretty much every official campaign-length adventure in 5e can be described as a filled with a lot of cool, disconnected ideas that need a lot of changes to work well at the table. That’s true both before and after Tasha’s. I would choose to run Netherdeep or Witchlight over Tyranny of Dragons or Princes of the Apocalypse any day of the week. I’d recommend pretty much any other official 5e setting book over the SCAG, except probably Strixhaven.
I dunno, maybe you feel different, but in my experience in discussions like these anyone that says “D&D 5e after Tasha’s is clearly worse than it was before Tasha’s” either isn’t actually familiar with most 5e books, is viewing early 5e books with nostalgia glasses, or is saying that in place of what they actually mean (they don’t like the Tasha’s rule updates, overall shortening of lore, the new inclusion policies or something like that).
Besides the new core rulebooks, I’m pretty sure the last 5e book I bought was Fizban’s, so maybe the quality nose dived with Planescape, Book of Many Things, and Bigby’s, based on my experience with 5e books, I just straight up do not agree with this simplistic assessment of the trajectory of quality in official 5e books’. And I think any simple dichotomies like “before Tasha’s good, after Tasha’s bad” are overly simplistic and inherently suspect. Reality is rarely so simple and people that want to reduce the complexities of reality often have an ulterior motive.