Exactly my feeling towards Eternal as well. I think you hit the nail on the head for Doom 2016. That game is just about near perfection as I can imagine it. The vibe, tone, atmosphere and combat all just screamed Doom, where Eternal just feels like it goes way overboard. What I love about 2016 is it kept the story extremely simple: Doomguy (Slayer) the UAC, Mars, Hell Portals. demons, lots more demons, and Hell. That is all a Doom game ever should be and all it needs to. This convoluted lore and page after page of codex reading is so inane and not Doom I honestly can't believe there aren't more people turned off by it. Do people really prefer to read pages of codex lore in their Doom game? It just goes so against what the original team at Id did with Doom and its minimalist story that it's just really jarring. I really enjoy the character of Samuel Hayden, and believes he acts as a perfect foil to the Slayer, but all this Seraphim nonsense and the Urdak storyline with him just kind of ruins it for me imo. He went from being a true Machiavellian yet simple to understand character willing to act in his and humanity's interests even if it compromised everything on Mars to this now extremely powerful deity with more pages of lore to read and backstory. I'm not even against having a complex story, but couldn't have Id just shown us this through gameplay instead of locking it behind the codex pages?
I'm right there with you on your feelings about Eternal, and having played through almost all of Ancient Gods P1 at this point I'm pretty disappointed this will be the direction Id will be taking with future Doom titles.
Edit: Also find it baffling the epic cliffhanger from 2016 was completely unaddressed save for a BRIEF reference in a codex page. What an opener Eternal could have been with you battling your way back to Mars to retrtieve the Crucible (which is also disappointing as it is a glorified 3 charge chainsaw replacement) from Hayden.