Originally Posted by
Duilliath
Agalloch is not nearly as known in the EU as you seem to think. By and large, they're an American band with an American following. In several decades of reading through various metal zines, both on- and offline, never once did I get to see any of the following references: "For fans of Agalloch", "the band is influenced by Agalloch" etc etc.
Much as it will probably pain you to hear, Agalloch is largely unimportant over here. I have my doubts about their importance over at your neck of the woods, but seeing how it's a fair bit harder to double check for me, I'll just let it slide.
One of the biggest names in metal, same as in any other type of music, means "one of the big sellers". I tried to find listings for their record sales, but didn't manage to. I seriously doubt they reach numbers close to anything a Slayer, Megadeth or Fear Factory achieve. Or a Dimmu Borgir/Nile, for that matter.
Additionally, their influence on the bands you listed is extremely limited. Quite a few of them started around the same time at or before Agalloch (though possibly in different bands). Furthermore, acoustic guitars or genre boundry-breaking instruments are hardly Agalloch's doing.
//edit: Let's back this statement up a bit. Agalloch released their first full-length in 1999. They formed in 1995. Alcest formed in 2000 and Neige was already active in the (black) metal scene before then. The people in Les Discrets had been involved with involved with Neige for a while already. With Woods of Ypres, Agalloch's influence is listed, but having never even heard that band I won't say anything about them. I will point out they're a Canadian band, as opposed to Alcest/Les Discrets.
Additionally, Agalloch themselves covering an English Neo-folk song on one of their releases already points towards a source far more likely to influence European bands directly. As to Black Metal meeting Folk, Ulver had done such a thing several years before Agalloch even formed. Again, they're far more likely to have influenced the entire scene than Agalloch.
The only way you'd possibly convince me of Agalloch being one of the biggest names in metal would be if they changed their name to something like Agalloch's Vital And Important Musicological Entertainment Studio. Which really is fairly lengthy and possibly one of the bigger names.