Good level design != how open it is and no other qualifiers apply.
Doom Eternal's somewhat linear levels are way better than something like GTAV's massive open barren nothingness. Or for something more similar mechanically, better than Serious Sam 1's endless bland rooms and open environments.
I was joking of course, but there's something to be said about the fact for a split second you could feel like you were navigating an actual place where you had to absorb your surroundings and check out a possible path.
I'm mostly ambivalent on this topic, because as far as I'm concerned the non-linearity is an illusion in games where there's no focus on long-term resource management (i.e. Doom Eternal, DMC) because what route you take in those games doesn't affect how the core gameplay plays out (i.e. the combat), as opposed to games like Resident Evil, Pathologic 2, or Metroidvanias where
routing itself is part of the core gameplay. Hell, the non-linearity in several olDoom levels tended to be fake as well, as the keycards and keycard-gates were usually placed in such a way where there's only one correct route to take to complete the level, and deviating from it just makes you hit dead ends and wastes your time (unless there's secrets to be found that way).
However, even though I personally don't care much for non-linearity in games that don't rely on routing, I don't consider fake non-linearity to be a bad thing. If anything, I would argue that developers should take this non-linear approach to level design where possible even if it is fake, instead of using the standard hallway-arena format. While it may not make the game deeper and just makes it harder to find your bearings, the act of trying to find your bearings in itself does require more player engagement than moving forwards in a straight hallway, and player engagement is exactly what you want. You still want your downtime so the player doesn't get exhausted from having to be 100% on their game all the time, but by making them figure out where to go next in a non-combat situation you can still keep the player engaged without it being exhaustive or putting the player to sleep with an empty walking section. Doom Eternal already gives you HUD markers on where you need to go next (that you can disable), so it's like not like people have to put up with aimlessly getting lost if they don't want to. Having to backtrack through previous areas allows developers to get away more with reusing assets, so it basically saves money as well. And more importantly, there are more people who really like to see more 'complex' level design (even if it is completely fake) than there are people who absolutely do not want it or even people like me who don't care either way, so going that route is basically a win-win unless the gameplay really does not support it (because there's a time limit or something).
