Humans are dominant on Khorvaire, which is where a lot of the focus of the setting tends to be (but it doesn't have to be - it's easy enough to run a game somewhere else if you want to, and PCs are expected to move around a lot).I know very little about Ebberon, but I've seen a couple of people raising facts and direct quotes from the books that seem to support the position that humans are the dominant species.
Your counter argument amounts to ad hominems ("This is starting to sound like desperation to be right.") and cries of "Nonsense!" If you're right, you're certainly not making a case for your position.
Humans appear to be dominant on Sarlona, but their society is actually controlled by Outsiders.
Dragons are dominant on Argonessen.
No one is dominant on Xen'drick.
Elves are dominant on Aerenal (which is a lot smaller than the 4 continents above).
Pretty much nobody lives on the Frostfell or Everice.
The Oceans have a strong sahuagin presence but it's not clear to me whether they dominate.
Khyber (the Underdark) is arguably not part of Eberron itself (it's more like a separate plane) and is full of monsters.
Whether Humans are the dominant species on Eberron as a whole is a matter of opinion.








