It took him 4 days in intensive care to asphyxiate to death due to a cold.
I am sorry for your loss, but that seems highly unlikely. Motor Neuron Disease is a disease of the neurons carrying motoric function (usually but not exclusively A alpha fibres) - it's in the name. For him to asphyxiate, he should have respiratory involvement, in which case that would be what killed him if he refused intubation and ventilation or tracheostomy. Alternately he could have had atelectasis of his lungs or such, but then he probably died from a viral pneumonia, not a cold. To clear secretions he would just need physiotherapy and suctioning, which he certainly would've received in intensive care.
Regardless, this is a good example of what I am talking about. MND is exactly that kind of rare disease that we would prohibit any treatment from being developed or appropriate care being offered, thus essentially predisposing them to Euthanasia. MND is an array of diseases, that we know little of, so every person lost limits our ability to learn about that specific disease, and decreases any incentive to develop ways to cope with it. In future a new person diagnosed would be shunted toward that path, simply because effective treatments or coping strategies have not been developed further - from decreased demand and subject.
Besides, this merely amounts to anecdote and appeal to emotion - not Rational argument. To be that, we require Coherence, and perhaps Validity and Soundness. I presume you aren't in favour of 17 year olds killing themselves for PTSD or depression; so the coherence of arguing suffering falls away if we have to rely on subjectivity, validity is lacking as we can't quantify quality of life aspects in relative valences, and thus the whole is neither cogent nor sound. I certainly understand why people feel it should be an option for someone suffering from MND, but that does not make it a rational argument that it should be, nor why it should be allowed for the terminally ill in general. The reason why such measures are slippery slopes to on demand death on flimsy grounds, is precisely because the arguments they rest upon are so vague and weak - thus legal challenges keep pushing them further, and exceptions keep occuring, until we reach levels no one ever intended. Further, the frog in the pot principle applies as people become used to it, and what they would consider abhorrent, then becomes acceptable, as in the Low Countries.
Personally, I don't think not-being can ever be seen as more desirable than being on purely naturalistic grounds, nor how we can judge something so negative to trump all other experience or potentiality of experience or potential alleviation thereof. The negative aspects for the medical fraternity getting involved here are alse legion, from abuse potential to ethics to personal. While I have compassion for the sufferers, I think this a step too far, rather than comprehensive palliative regimens.