Yeah that DM role is way more than just being a good mobile tackler that protects the back four. You can just play 3 at the back for that with the central man being a sweeper style defender.
At a team like Arsenal, the defensive side of the role might actually take up less of the overall job than the ball playing side of the role.
DM’s are still proper central midfielders that can keep the ball under pressure, retain possession and can supply. How can we expect all that from an orthodox CB?