+130 000 peak concurrent players on Steam during the first weekend, suggests well over 400 000 copies sold on the first week for Dark Souls 3 PC version (the vast majority being digital download ofc). I think I saw Steamspy had it at 600K even, first week.
I get your point but I think the games are doing quite good considering the niche label.
According to vg console sales are:
Dark Souls: 3.04 mill
Dark Souls II: 2.74 mill
Dark Souls III: ? (fastest selling Bandai Namco title in the West)