> How could it be based on anything else?

The point is that the capability is measurable but the capabilities we consider to be essential are based on normalcy and thus effectively arbitrary. Eugenicists make the argument that evolution demonstrates that the classification is not arbitrary because deafness and blindness confer measurable fitness disadvantages, but they don’t actually bridge the gap of deriving an ought from an is.

> Obviously?

If the answers to these problems are obvious to you, perhaps you’d consider writing a book instead of participating in a discussion forum. I would encourage you to review the site guidelines.