One has to remember that souls is a different genre from hack & slashers like DMC, God of War, Ninja gaiden, & Platinum's games. Souls didn't get mainstream until Dark Souls, even Demon's Souls sales pales in comparison to DMC & God of Wars and only did
1 million copies. Nioh too did about
2.5 million in 2 years, so DMCV has certainly already passed it.
If Dark Souls never gained so much hype for being "really hard game you died a lot" it would've never gained the amount of fans it has now and would've been pulling similar numbers to Demon's Souls. It alsohelps that FROM Software's games are relatively cheap but still sell very well, they can churn them out.
In turn, FROM Softwares current games have a different type of appeal than hack & slashers of old and have only been growing in popularity from last gen to this gen while hack & slashers have severely declined in popularity.
This goes back to what I said about the biggest game in the sub-genre, God of War, leaving the sub-genre to go to a genre with even bigger mass appeal. That's because God of War Ascension didn't sell well, not even close to God of War 3, and Sony weren't sure where to go with the series until the visual, story and genre shift.
In 10 years, you only had one DMC game and that was the reboot which fans hated and didn't reach sales expectations either, ninja gaiden pracitcally died and it too had low sales and outside of that only platinum regularly released hack & slash games but none of platinums games are high sellers. Automata is their best selling game, even Bayonetta which built up their reputation only sold 1.5 million copies for lifetime which is why you didn't get Bayonetta 2 until years later when Nintendo paid for the games exclusive development.