I do think the issue re the value of life is a personal one. But only personal to oneself.
I think it is possible to decide that your own life has no value, and tbh I don't think anyone else can judge that as only you know how you feel. However I don't think that it's possible to make that judgement on behalf of someone else, so to say that someone else's life has no value, because you can only really judge that from your own opinion.
Dementia is totally different to disability because dementia is a degenerative, progressive illness, the outcome of which is always death. Whereas becoming disabled must be a terrible shock and take immense adjustment, the outcome is usually a different type of life, but it is still life.
If I were diagnosed with dementia then i would end my life before it got to the point where I didn't know who I or my family were, and while I was still in a position to be able to do it myself. But I see that as being different to becoming disabled, because it's not a question of coming to terms with it, you are going to lose your mind, you are going t die, and I think that it should be allowed to happen with dignity.
But this man was young. He had only been disabled for 18 months, and he most likely had not yet actually come to terms with the fact. And maybe he never would have, but we will never know that now, because those who were supposed to be supporting him did so in the wrong way IMO. how much convincing do you thinks the parents might have needed? 18 months isn't long to convince someone to help you kill yourself after all.