Our understanding based on the court ruling of the circumstances of Kay Carter was that she was in a wheelchair and needed assistance for most of her daily functions, so her mobility was quite limited.
It's not really a provision of paragraph (b) that speaks to there being a change over time. It can be a state of advanced and irreversible decline and capability that occurred all at once, for instance. It isn't part of the requirement that there be continuing decrease in capability; it's only required that the person be in an advanced state of decline.
It might also be helpful to note that with all of these criteria, including what is an incurable illness, the decision-making on these elements has been left in the hands of medical and nurse practitioners. What is or is not an incurable condition may depend on a variety of factors, not just necessarily the name of the disease, for instance. There might be implications relating to the accessibility of treatments
However, if we come back to the “advanced state of irreversible decline”, it's also something that can take into account a whole host of considerations. A person might be in an advanced state of decline due to exhaustion, due to fatigue, due to frailty. Again, it's designed to be flexible in order for the medical and nurse practitioners to take everything into account.