[AnIdiotOnTheNet]

As another comment pointed out, modern shooters have actually become simpler overall since Halo popularized the limit on how many guns you could carry and regenerating health.

What's actually wrong, I think, is that most FPS's just don't have interesting gameplay. They've failed to learn the lesson DOOM taught: it isn't about the weapons and the shooting, it's about the movement.

What makes DOOM so fun isn't the aiming and pulling a trigger bit, indeed, DOOM can only aim in one dimension anyway. What makes DOOM fun is the way the enemies' behavior and attack patterns interact with your movement and positioning, and how your current weapon's mechanics are best exploited. It's about a complex and dynamic interplay of positioning and distance. Taking on multiple enemy types at once means solving a multiple variable problem in real time. Doom 2016 did a pretty good job getting this right.