It's a mess:
Doom 64 is technically Doom 3 (Sequel to Doom 2)
Final Doom is an alternate sequel to Doom 2 (also Doom 3 then)
"Doom 3" is technically a reboot
Doom 2016 is technically Doom 4 (Codex entries hint it's a quiet sequel to Doom 64 aka "soft reboot")
Doom Eternal is technically Doom 5 (unless ID cement that Doom 2016 is finally a full reboot)
A hot mess indeed...
That whole list is a giant mess on trying to number any of it, so maybe that's why they rebooted not just how the game worked in 2016 but the name as it's a mess. Funny we have 'DoTheMath' attempting to do the math with iD.
The fascinating thing to me is how different Doom 2016 and Eternal are. The existing game plays like it's cracked out of its mind, always notched up to 11 having to just not be tactical, just run in and kill crap and use that chainsaw a lot for all the instant ammo/health refill explosions since they no longer leave enough in the stages anymore. Then you saw the demo of the new one, it's obviously slower, more calculating and bits of discovery to it which made it feel like when the Quake franchise first came out 20 years ago. I really hope it works out well for them and if I'm right in how I see it, it goes forward that way. Doom 2016 is too brainless and fast for my taste, but I still like to open it up every rare often to shoot things but the pacing sucks. It's just rampantly fast on kills, much like some mass bot fight or online multiplayer deathmatch.
I imagine that Eternal will also be frantically fast like Doom 2016 is... I think it's too soon to make that call, just from the available demo. My two cents anyway. I loved the 'in your face' aspect of Doom 4, but it seems that I share your sentiment, in that the 'glory kill' dynamic is a necessity to progress in the game. That upset me, considering ID said it wasn't going to be necessary, but imo... it was necessary. Mind you, I played it on the Playstation 4. If I played it with a M&KB on the PC, it would be much easier.
My MAIN gripe with Doom 4 - and I'm surprised I haven't seen many other Gamers mention this... is how absolutely LAZY and chickenshit of a move it was, for them to not change the Placements up for the varying difficulties. They went the Noob route and simply add health to the NPCs, and also how much damage they do to the player. I was extremely disappointed to learn that, after beating the game and wanting to play it on a higher difficulty. Absolute dingbat move on their part and if I was a journalist, I would lambast them with that point until I was blue in the face. They need to work for a living, and do unique NPC placements for each difficulty. That is a standard that should be respected, but I digress....