Isn't that his point? 500 years ago, nobody would be willing to hire Steven Hawking. Period. He simply isn't capable of doing anything resembling what they would have considered work, even with the use of the modern tools he uses to interact with the world. Today, however, he is able to be a completely productive member of society because the work we need to do is completely different – and the work we need to do 500 years from now will unlikely resemble much of what we do today.