If graphically you don't add more details that weren't previously possible then what is the point of a remake? If you look at the original Boletaria concept art for DeS it looks nothing like DeS PS3. So clearly not because that's how they wanted it to look but because what was in the concept wasn't possible at the time. Miyazaki himself even said that it was exciting to see Bluepoint research and do things that they couldn't do at the time.
The fog in SH2 added to the atmosphere and was in fact a gameplay mechanic and relevant story scene. Removing fog would be stupid. Adding details to a palace while maintaining the same atmosphere is not.
Which itself had departed from lore and concept art due to technical hardware capability and budget. Point is that a better detailed palace isn't a big deal that ruins some setting or atmosphere, that remains intact. Unlike the removing fog in SH2 example which would have meant story and atmosphere changes.
Do you think Boletaria in the Remake is simply Demon's Souls with "more details" or have they massively changed the architecture? The Last of Us Part 1 is an example of the same environment with more detail. Just because the PS5 is more powerful than the PS3 doesn't mean they have to make everything gothic and throw towers everywhere in TLOU just because they can.
Boletaria to my eyes (I may be technically incorrect here) looks more based off what I'd see on a norman castle.
You can argue Demons Souls remake looks better, but it's nonsensical to say it retains the same architecture as the PS3, but adds more detail. It's a whole different design philosophy. I'm not debating opinions on what looks better here I'm stating the obvious.
Also referring back to concept art to prove your point doesn't matter either. A lot of concepts don't make it to the final game. The end result is what matters. You can argue the concepts will enhance the game if included, but once again that's an opinion. Like I could say Tomb Raider would be better if they stuck to having a male as the lead character and the recent Remasters would have benefitted from replacing Lara with the original male.
What you're doing is essentially deciding what works and what doesn't work based off your own opinion. It's fine to have your opinion and I'm not debating that. I'm just saying Boletaria in the Remake doesn't retain the architecture of the original.