Wallerstein argued that any mercantile system, call it capitalism, call it whatever you want, advances by liberalism but that the individual members of that mercantile system advance by conservatism. The winning practice is declaring that everyone is equal, therefore the people who have the most success are simply the most meritorious.
A side-effect of this is those who enjoy the most rights under the de facto racist de jure equal system are, by inspection, low-achievers. They are not threatened by the wealthy because they have already won. They are threatened by those the liberals hold up for equality. Thus, "great replacement theory" is just another name for a countercultural phenomenon that dates back to the diaspora. Pretending this goes back to F. Scott Fitzgerald or some shit when it was the underpinnings of the Protocols of the Elders of Zion, the Pogroms, the Witch Hunts and the Diaspora is myopic in the extreme. "Replacement" was a principle argument against suffrage. Here's Great Replacement Theory during Reconstruction.
For that matter, "Replacement theory" was a Republican argument against the expansion of slavery. If slavery expanded to the new territories in the USA, no free man would be able to compete against slave labor. These arguments, of course, made it into Mein Kampff, only against Jews and "waste people" instead of slaves.
I was unaware anyone made the argument that Great Replacement Theory was a "fringe" idea. The "fringe" part is saying it out loud. That's been the principle complaint since Trump - he says the silent part out loud, which gives everyone else permission to do the same. In order to function in modern capitalist society, you have to pay lip service to equality or else you lose the game. The current crop of populists don't have enough advantages in that game so they're trying to play another.