I think the concept of symmetrical game design vs. asymmetrical game design is at the core of what we’re discussing here. The old-school approach treats NPCs as CPU players (where the DM acts as the computer) in a symmetrical game. The modern approach treats NPCs as game pieces controlled by the DM in an asymmetrical game.
Ironic, considering how player vs. DM mentality is generally seen as a hallmark of old-school play. But it makes sense if you assume, as is often the case in asymmetrical games, that the player in the role of DM has a different goal than the players in the roles of PCs do. In old-school play, the PCs and the NPCs are both playing with the same goal of defeating the other team. In modern play, the PCs play with the goal of defeating the DM’s units, while the DM uses their units to play with the goal of making the PCs heroes.