Prepare to Die was very much built into the game.
I think that you are essentially right about that. There is a difference in the design philosophies between Demon's Souls and Dark Souls.
Simply put, you are not only expected to die in Dark Souls but also supposed to. And while there are often ways for the most careful players to avoid many of the intended deaths, that intention is reflected in both how the game is designed and how it was marketed but also how death is essentially used as a tool. It's not supposed to be a big deal at all that you do die on occasion (which is a possible reason for why the mechanic of losing health upon death was removed). How you react to that design philosophy will undoubtedly have an affect how much you like or dislike Dark Souls. With Dark Souls specifically the expectation that death was something that would and should happen still worked for me, but since then the series has gotten stale.
I remember heavily disliking the marketing that they started to use with Dark Souls. I thought that it missed the point entirely and perpetuated the belief that the series was all about difficulty. There was definitely a shift in the series' design philosophy.
To an extent I do have to disagree with the video about boss designs. Demon's Souls does have memorable bosses but whatever else might be said about them, as fights they are not as enjoyable as many of the ones in the later titles. Micolash was not a fun fight either, which only becomes more apparent in low level runs.
While Dark Souls remains my personal favorite game in the series, I think of Demon's Souls as the best one. Whenever I play it I get the impression that it was exactly what From Software wanted to make and not much else. In the end it happened to work out brilliantly.