That's not really steelmanning. You can't steelman a position by saying it's not the real position but it dupes the rubes.

The great replacement theory is the theory that there is an intentional effort to dilute or replace the capital-W White, meaning the historical English/Scottish/Scotch Irish, population of the US, with immigrants and former slaves, and it usually involves a part that says that it is being done to weaken the country against its international competitors. A third part that is usually involved is that the process is being facilitated by and for the benefit of people like "international bankers", "cosmopolitans", "elites", etc., terms which have an antisemitic history.

To steelman it, you would have to steelman at least the intentional dilution part. Not just to say that it is hard to meet our demand for labor without immigration but that someone is coordinating it. Further, I don't think it has any meaning without the part that says it is being done to weaken the country, which you would have to show that not only would it weaken the country, but that is the intention of these coordinators.

Without that, you just have a demographic argument. If "Whites" do not have many children, and the population would otherwise shrink as a whole, while immigration is needed to satisfy demand for labor, then their proportion will shrink, but it is not "great replacement" without it being intentional/directed.

It is what I consider steelmanning.

Not of the conspiracy itself (I'm not interested in that, since the literal version isn't even well agreed upon by most of its believers) but of the observable pressures that make the conspiracy attractive.

I think I could convince an average believer in the great replacement theory (who would be a casual believer that doesn't know many of the specific details you've listed at length of the "official" version) that my restatement of the issue is what they're actually concerned about. In fact, I have had productive conversations with right wingers who express such a casual belief in this theory by telling them what I've written here in the comment you're replying to.