The point is that physical impairments, while real, shouldn't matter all that much. We will all be physically impaired to a greater or lesser extent as we age. Social impairment however matters a great deal, because it's next to impossible to live a healthy and fulfilling life if you can't relate to others on a reasonably equal basis. Of course trying to enforce this on everyone else with "person-first" language is a total non-starter, but the basic insight found in the "social model" is quite valuable and in fact something that classical authors were already familiar with.