No surprise that forums are full of comments over previous versions of Doom as compared to the upcoming Doom Eternal.
The original Doom and Doom 2 were very linear with small maps due to the very limited systems available. Doom 3 had much more memory and better graphics cards available but it retained the linear gameplay.
Doom 2016 made a move to a semi-open world which was a break from the old and heavily scripted releases. Doom 2016 allowed players to move around the game extensive world freely.
Doom Eternal has a new engine which better leverages modern video cards. The game has more monsters to shoot but once again its is not as linear as the classics. Bethesda and id Software have the legacy in mind when they designed Doom Eternal before embarking on development.