"That Stephen Hawking bloke earns his own living, therefore benefits should only be given to people who are more disabled than he is."
Yes, that Stephen Hawking bloke does earn his own living, and all power to him for that. However:
1. He is a bona-fide 100% genius, and was already recognised as a genius before his illness was affecting him.
2. Quite possibly because of that genius, he has had access to custom-made adaptive and assistive technology FAR above and beyond the norm. Professor Hawking was using technology in the 80s that is not necessarily available to people with the same condition even today.
3. If the genius aspect was removed - if instead of being Professor Stephen Hawking, PhD, CBE, FRS and however much else of the alphabet you feel like adding, we just had Steve Hawking with seven mediocre GCSEs from the local comp and a bronze swimming certificate - how employable would he be? If the man who holds the workings of the universe in his head were to express an interest in coming to give a lecture at your nearest college or university, it's a fair bet that they would scramble to provide wheelchair access to as much of the campus as possible and make every other adjustment asked for in terms of allowing extra time, ensuring appropriate parking space, and whatever else is in his 'rider'. Would they do the same for someone who had applied for the minimum-wage caretaker's position?
Professor Hawking is a remarkable man and as such he is the exception, not the rule. The only possible answer to "Stephen Hawking has a job, why don't you?" goes something along the lines of "Stephen Hawking has written several best-selling books explaining scientific mysteries which have baffled the finest minds for centuries - why haven't you?"
It's one thing to aspire to the achievements of the most amazing people ever to have lived, but quite another to take them as a benchmark for what is expected of us.