'What has how long you live got to do with it? It is an illness, illness isn't treated because of how long you will live is it. Some people with dementia don't last long and some people with cancer get treated for years.'
The difference is most cancer patients don't wind up requiring 24/7 care for years and years. Plenty of them are able to care for themselves at home or with minimal support and even work during some of that time. Some wind up with significant disability and are treated the same as anyone else with significant disability who requires social care, the difference is the requirement of social care, that's the big issue here, as has been pointed out by numerous posters. Rates of cancer are rising, too, because believe it or not, the biggest risk factor for cancer is age, the body just doesn't do as good a job of proper cell division or the immune system of spotting rogue cell growth.
There are even age cut offs for some treatments, not because of discrimination but because of the fact that survival chances are minimal to none. Not to mention, plenty of treatments, even for children, are not available on the NHS.
You may also, if you are not even adult but of competency on the Gallick scale, elect to end treatment.
You cannot do this if your mind is not sound due to dementia or any other similar illness.
This is why I believe in Living Wills, medical POA, and legalised assisted suicide, including for those who make such a decision when they are of sound mind in the event that they become mentally compromised by illness, as it often happens with some conditions that the patient is unable to recognise his/her own deterioration until it is too late, or at the very least Living Will to allow withdrawal of medical treatment.
Part of the problem is inability and distaste at discussing one's own mortality or even recognising it. Hence, threads on here with the overwhelming tone that if you 'do it right' and engage in certain lifestyle behaviours, you will not suffer disease. Loads of awful threads about bereavement, or labelled 'morbid'. What's needed is more acceptance of our mortality and discussion of it.