The fact that there was any form of plot at all was unique in the action and shooter genres.

No. It wasn’t.

There is this mindset that all that existed was DOOM (which actually did some interesting things narratively).

Marathon 1 came out in 1994 and built on Pathways into Darkness (1993). It came out the same year as System Shock and the year after CyClones (woefully underrated). Both of which also heavily relied on text bits but also, in my opinion, did a much better job of tying that narrative into the level/encounter design itself. Something Marathon… kind of wouldn’t really do until Infinity in 1996 where there is even more competition.

It was the first major release with free look and being able to aim up and down at all.

Again, CyClones and System Shock

Plus reloading weapons, dual wielding weapons, weapon models visible on the player in multiplayer,

“Tactical” shooters had already existed and I want to say there were a few DOOM Engine games that had reloading by this point?

Weapon models? I doubt it, but sure. Voice chat? Sure? That sounds real fun over sub 56k internet.

None of which changes Marathon classic mostly just being a “generic” FPS with a wall of crazy lore bible used to make the log entries.

To be clear: I LOVE the Marathon Trilogy. But if you actually look at what the games were, rather than what we wanted them to be… they were great writing, awkward level design, and decent shooting.