> So, if you wanted a capital W whites, that likely would be ... a specific cultural and geographical unity of white-skinned people, but by far not all white people.

Thing is, white immigrants' national identities were largely, um, white-washed in the late-19th thru mid-20th c., as part of Progressive ideals that back then were quite racially slanted, and included such things as eugenics. German immigrants were especially affected by this for obvious reasons, but not only. So the argument for capital-W 'White' is hardly any weaker than for capital-B 'Black'.