Firstly, like I said, back when the DLC first came out, a ton of people were divided on that area in the Dark Souls 2 OT. Obviously, I was one of the people that liked it. Really love the area.
Secondly, the Dark Souls 2 DLCs are the same isolated levels that all the other DLCs are? I mean yes it's a contiguous level but it's not like they're forcing the area to fit somewhere in the core game. The Sunken King is a giant puzzle labyrinth in a ziggurat. What's not to love?
Lastly, the Demon's Souls bosses are more than just standard enemy fights. That's my whole point about them. I think it's commendable and awesome that they are all mostly gimmick fights and I prefer it that way. Starting with Dark Souls, most (in practice all but two or three) just had you slug it out with a single enemy like you would any standard mob. Yes, of course, fighting Asylum Demon, Taurus Demon, and many of the other bosses aren't just a hollow dreg, but you are still approaching them with the mindset of a combatant in that way.
But Demon's Souls bosses are so much more than that and it's awesome. You've got Fool's Idol which is a tricks and traps boss. The pacing of that fight is completely different from Armor Spider that has you running back and forth in a tunnel which is completely different from the pacing in Maiden Astraea's fight which is extremely personal and intimate (and thus in extreme opposition and juxtaposition with the entirety of the Valley of Defilement).
When I'm talking about OoK movesets and any of the Hunter battles in Bloodborne or any of the standard typical boss fights in Dark Souls, I'm thinking of them from the perspective of fights like Flamelurker and Allant and Allone and Fume Knight and (now) the actual Hunters in Bloodborne which are the exceptional top tier "slug it out with a dude" boss fights in the series.