Harvard defines "disadvantaged" as 80k salary per year (roughly 65th percentile), and according to its own scoring scheme, only 30% of its minority admissions fall into even this very generous conception of "disadvantaged".

So you're just wrong, if you're talking about socioeconomic opportunities. Harvard themselves scores them as doing fine.

Then you go on to attack a straw man with the very poorest backgrounds. You can admit people with economically diverse backgrounds without having to select only the very poorest. (Indeed, the thing Steve linked to regarding the 70%-not-disadvantaged number makes exactly the argument that student quality would not be substantially affected. And, further, given the large test score disparity that a lot of historically disadvantaged minority students come in with, if this was an issue it would be an argument against affirmative action!)

You either need to come up with a better argument or need to change your position.