From the current research, the best conclusion I've been able to arrive at is we still don't really understand ALS. My opinion is that there may be several distinct motor neuron diseases, each with accompanying cause[s], currently grouped together in what we call ALS. There are notable differences in early vs. later onset ALS, slow vs. rapid disease progression, initial primary symptoms, genetic links vs environmental factors, etc. (I'm not a doctor or professional academic in the field, but I've studied ALS research extensively - a relative was diagnosed and died from it, suddenly, in their early 60s.)

Stephen Hawking is definitely inspiring, with a few unique points - he had an early onset of the disease, slow disease progression, and extensive resources/access to the most advanced care. With mechanical ventilation, caregivers, sheer will, some luck (basically, not contracting a fatal case of pneumonia), tracheotomy and feeding tube, etc - life expectancy with ALS can certainly be extended far beyond what was once thought possible though.