It seems to me that we HAVE accepted that on the social level and so are paying that growing percentage of the population so that it doesn't starve (or turn to crime, etc.).
I think we're disagreeing about the use of 'accepted' rather than the facts: I mean that the media consensus is not that we have 'surplus population' who should be paid to not starve or cause too much trouble, but that we have a growing number of people with disabilities, and that the labeling as 'disabled' is a significant portion of why the voting public is willing to spend taxpayer money on them. I understand you to use 'accepted' to mean that SSI is still funded and protected by public opinion against significant cuts.
I think there is some definition fuzziness here. We started with "percentage of the population ... not able to contribute productive work", this mutated into "surplus population" which is not the same as "people with disabilities"?
A resonable (economic) definition of "disabled" is "not able to contribute productive work"...
Do you have in mind what Tyler Cowen calls ZMP (zero marginal productivity) workers?
A post from Gregory Cochran's and Henry Harpending's excellent blog West Hunter.
The commenter Ron Pavellas adds:
The Wasserman Test.