The increasing focus on bullshit bosses is part of the decline.
I enjoyed Sen's Fortress for the fortress, Iron Golem was a good boss after a great level, same with Blighttown and Quelaag etc.. nowadays everything in From games is banal except the bosses, which are bullshit.
Excuse the hyperbole. Still good games compared to the rest of the industry.
Am a hopeless lover of From Software but I agree with you. The series at its peak, for me, was about excellent and atmospheric level design topped off with a good boss. People meme on the difficult bosses like O&S, Malenia and Gael so the devs think people want more of that because some big mouths won't shut up. Its the same bullshit where people act like the draw to the series is the difficulty.
Also I'm glad you mentioned Blighttown becasue it was a damn good level that people moan about too much. It sucked on the original hardware due to slowdown but once that was fixed it easily became one of the best zones in the game. The swamp = bad Dark Souls meme is trash. Lets look...
Demons Souls - Valley of Defilement. Peak incline, best swamp level ever.
Blight Town - Awesome, great verticality and fun in general.
The Gutter - Kind of annoying but I enjoyed the atmosphere plus any level involving the torch mechanic is great in DS2.
Farron's Keep - I liked this level somehow but I don't know why.
I agree that the world and level design in Dark Souls 1 is what made the game, all though i wouldn't go so far as saying the boss design didn't matter.
However, if you are going to make an argument that loss of focus on level design is "decline", wouldn't that apply to Bloodborne as well?
Because i think that's when the "decline" in question begun, especially since a lot of what was in Bloodborne got carried over into Dark Souls 3.
At that point, one might say something like "well, Bloodborne was a different kind of game, didn't need big interconnected maps like Dark Souls 1", but then, we aren't talking about decline specifically anymore, but different and apparently legitimate directions. In which case, big focus on bosses and less focus on level design is appropriate for games like Bloodborne or Sekiro, but not Dark Souls.
When i first played Dark Souls 3, i made a similar argument, that the focus on boss design at the expense of level design was taking the series in the wrong direction and i even blamed Bloodborne for it. Turns out the issue was that they were super lazy when they made Dark Souls 3 which was really just a Bloodborne total conversion if you really want to get down to it.
Now compared to Dark Souls 3, Elden Ring was a return to form if we consider the legacy dungeons, and it's only the gargantuan open world filler that gets in the way of that special Dark Souls 1 experience, but then Elden Ring is a different beast from either Bloodborne or Sekiro so i'm not sure we can talk about a single, general decline here that basically amounts to bosses at the expense of map design. As much as we all hated the open world, it isn't exactly that. Elden Ring also offers a great deal of customization, build possibilities and lots of "toys" to play around so again you can't say there's a general trend to turn Souls into Bloodborne. Apparently, Dark Souls 3 was a single case here.
This brings us up to Armored Core, because this is a game where the kind of Dark Souls 1 style of world building wouldn't fit. The only way you could expand on the map design here is by making them larger with multiple militarized objectives etc (and i certainly wished they had done that). The introduction of Sekiro bosses into the AC formula is again a different matter than the focus on bosses at the expense of world design that one might see as a degeneration when applied to Souls. I don't know how elaborate maps were in later Armored Core entires but i can't imagine the design was much different than this.
