Not sure what you're getting at really? You're fighting demons almost constantly in Doom Eternal and just like Doom 2016 there is some pause between battles where you explore stuff. I fail to see any radical change to the gameplay loop in Eternal, it's still very much about fighting demons with 5 minutes of pause in between fights where you focus on movement and exploration.
I think it's telling that, early on, one of the most common criticisms was that you didn't feel like the Doom Slayer, like the object of fear of demons and all.
Doom 2016 worked as a "regular power fantasy" even on Ultra Violence (didn't play Nightmare in that game, only in Eternal). If you wanted you could really explore mechanics like weapon switching and stuff like that, but as far as just beating the game goes, you can pretty much do whatever and it'll work. As long as you keep moving and shooting, you're good.
Eternal is far more demanding on Ultra Violence and Nightmare. in order to live the power fantasy in Eternal, you have to earn it. You need to play in a way that will sell the idea that you're the Doom Slayer, that you're worthy being feared, basically. So you have to exploit weaknesses, especially early on. You have to use the chainsaw, you have to switch weapons, and so on. The game opens up a lot more from the moment you get the Meat Hook, but it's still quite demanding.
Things you could do in 2016 if you so desired, if you wanted to dig deeper into the combat system, are part of the regular gameplay loop in Eternal, and some people find it a bit too much, it feels overwhelming to them.
