The thing with nuDoom and Eternal is that it's designed from the ground up for arena combat, which means it's shifted from long-term to short-term resource management. In olDoom, levels had a constant tension to them because every hit you took and every shot you fired mattered when the amount of medkits and ammo in a level were finite, and you wouldn't know your first time around where the items were or if there were any at all. Exploration and combat were tightly intertwined. In nu/Eternal, every enemy is a potential medkit/ammo pickup (to make extended fighting in arenas tenable and give the player a reason to move away from islands of items that would be their only means of regaining HP otherwise), so the dynamic of tense exploration can no longer apply.
Because item pick-ups are no longer worthy rewards for exploration when you have Glory Kills and such to regain resources, the only long-term rewards the game can offer is through upgrade points to permanently upgrade yourself. Because the combat in nu/Eternal can only get any tense in locked-off circular arenas (since you could just S
1 your way through otherwise like in many olDoom levels), everything inbetween arenas is just downtime, one consequence being that the exploration segments were boring as hell as you would only every fight a small amount of enemies in them. I think the game should have removed exploration entirely in favor of non-stop back-to-back arenas with some occasional cutscene breaks, though I guess that's a minority opinion. In Eternal id's solution to this problem seems to be MOAR PLATFORMING. You could go back to old-school resource management and level design, but in the process you'd have to change everything what makes nuDoom unique to make it actually work. And since there's enough custom olDoom content to keep me occupied for a century, I'm in no hurry for a new Doom game that does what olDoom does but slightly different.
Great point about long term versus short term. I'm not a diehard purist so I think it should be a combination, maybe 50/50. I like the old system a lot but in very long and difficult WADs it can be draining. Ideally, GKs should always give you a fighting chance and keep you alive until the next pickup.
I disagree about platforming not killing you. I've always thought that Doom and Mario had a strange connection due to the importance of movement and it seems the developers acknowledge this as well. Imagine what Mario would be like if you didn't die when you fell. To some extent I understand why they did it but at least they should take away a life from you. As for Argent Tower, I thought it was very well designed. The first time I went through it, I never died. I DID die in it in a couple of Ultra Nightmare playthroughs (which was very painful). If you are correct and the platforming ends up being more boring that it seems to me then you have a point.
I also disagree about areas between arenas in 2016. To me it was one of the highlights of the game was the meaningful exploration (especially early on). For me, finding interconnected areas like that yellow keycard secret in the second level or the entirety of Foundry is one of my favorite things to do in a Doom game. In all Doom games, I love the downtime to do some exploration. It is for this reason why I want GKs to be toned down a bit in favor of the old system: so they can make pickup secrets more meaningful. They mentioned they are taking steps to reimplement modding. If true, I will definitely make my own balance mod at some point. Aside from this, there were some really cool incidental combat moments that I really enjoyed.
In Foundry, there are a couple of Hell Knights that spawn quite far way from you after you do specific things. On one of my first Ultra Nightmare playthroughs I totally forgot about one of them so I go into the small area before the final arena, getting ready for it, when all of a sudden I hear hoofsteps far away. They get louder and louder and suddenly the big door behind me opens, revealing a Hell Knight. I didn't have full health, no chainsaw ammo left, and and no plasma ammo to stun it so it bitch slapped me and stomped on my face. It was honestly one of the scariest and Doomiest moments I've had in a game. It was so unexpected. It also worked in part because GKs are less powerful early on in the game and in Ultra Nightmare, each pickup makes the difference between life and death. I really hope there are more moments like this in Eternal.