I haven't played demon's souls. If they port it to PC (they won't) then I would buy it, but I would rather buy a PS4 to play Bloodborne than buy a PS3/5 to play demon's souls.
I got demons souls for free on ps plus and it’s definitely not worth 70$, I haven’t played the original but there’s just so many dumb features, like all items in your inventory count towards equip load
dark souls has a similar feature, where you can only hold 10k pounds (I think it's 10k) and it's used in a item dupe glitch. Never tried it myself, but it's supposed to work in the remaster as well.