I just did the Cathedral of the Deep in DS3. It took me hours to get through the Cathedral. I beat the Deacons of the Deep in 2 tries and that intiall death is because I got greedy and surrounded near the end of the fight so I couldn't back out to restore stamina and heal.
The difference with DS3 though, is that there are more checkpoints and shortcuts. Demon's Souls literally only has a point at the beginning of the level and the boss fights, so if you die to a boss you have to go all the way back every time.
Hell the 4-1, and 5-1 areas dont even have shortcuts in them (that i could find), it's like the level designers had to rush to make new areas towards the end of the game. Which makes sense because of the budget the original had.
It's one of those Worts that Bluepoint stayed true to. Through Demon's Souls you can really feel a lot the shitty design decisions that made it one of the worst Souls games in almost every aspect. However there is a charm to it still and it's kind of awesome to see how everything evolved into later Souls games.
That being said, its 2020 and a new generation, you'd think Bluepoint could change SOME of the shit that sucked dick just to make a better introduction into the series for new players.