I believe the enemy limit in Doom 2016's campaign is 15 monsters, not 8
That's still pathetically low for a Doom game.
It's also kind of odd considering how well optimized the game was. I feel like it could have easily handled more than that.
Sure it is when compared to the majority of PWADS but it's not so low when you compare it to Doom 1, which is what it's largely based on. Besides, the balancing of the game prevents absurd amounts of monsters. Most demons are individually a lot more dangerous and agile than their classic counterparts so having 50 at a time would overwhelm the player. Now I would like to see more, especially towards the end of the game but I don't think more than 20 at a time is necessary.
I'd say it's loosely based on Doom 1 rather than largely. Still, if you go by the average number of monsters at any given time, the difference is significant.
I'm not saying it needs to match the numbers from the original games though. I'd just like to see at least slightly more monsters on screen and spread out more realistically as opposed to the arena style.