I think we still have a long way when it comes to advocacies for PWD’s. I’m not sure if the answer is to further divide the group according to a diagnosis because it will just become complicated.
I do agree with you though that the seats have to be reserved for those who need it most - whether it’s the older person with difficulties in balance, the physically impaired, or the invisible illness (whether it’s a mental illness like depression or some invisible physical illness that contributes to extreme fatigue).
I can only think of improving our transport system (more efficient and reliable trains, more seats, etc) as one way. And to advocate being more mindful. Kung kaya tumayo, tumayo. If not, reserve that seat for someone who might need it more than you.