These are some guesses:
These are now used so routinely that we forget they are life-extending treatments. In the past, many people, throughout life but especially in old age, were carried off by overwhelming infections - especially pneumonia, but also things like cellulitis (skin infections), septicaemia (blood infections). These are now unthinkingly nipped in the bud by antibiotics, with the result that people survive to get the various types of dementia.
Many old people lose their appetites and would not spontaneously feed themselves adequately - they would naturally waste away to the point of being easily carried off by any unusual physiological stressor. Nowadays there is a combination of ready prepared food, food brought to the elderly, and food being put in front of old people and them being encouraged to eat.
Cold stress means that core temperature must be maintained by increased metabolism and shivering - further exacerbating the problem of self-starvation and rendering individuals vulnerable to any further stress like an infection. Nowadays central heating is normal, and houses are kept well above outdoor temperatures.
Even nowadays, an elderly person's life is often terminated by a fall and broken hip or other bone (alternatively, they may be saved but dementia becomes evident from that point - perhaps due to anaesthetic and drug effects). In the past these would not have been treated effectively and would have accounted for more.
In nomadic hunter gatherer conditions, seventy years seems to be about the limit of lifespan for most people. In agricultural conditions this was extended in some people who were looked-after in sedentary (stationary) societies - and who were prevented from injuring themselves and protected from infections.
Nowadays, many more people in the developed countries are kept alive by the above means - plus others including resuscitation and life-support, intensive therapy, advanced vascular surgery (including heart surgery) and life-extending drugs and surgery.
It is striking that even in the early 1980s, sixty was the maximum age limit for the (cutting edge) coronary care unit which I used to cover on-call - nowadays people routinely get heart and vascular surgery up into their eighties (plus 'heroic' cancer resections and chemo/ radio-therapy), even when they already have dementia. (And such treatments themselves often induce dementia - although this is seldom admitted, noticed or remarked)
It is in this context of routine and unreflective life-extension far beyond the natural lifespan and regardless of 'quality of life' (because all this life-extension now regarded as a 'human right') that the intellectual elites of the UK are pushing and pushing for a system of humane murder by medical means.
I saw an advertisment that an example of this is going to be televised by the BBC, as part of the propaganda for... What is the catchphrase? Ah yes... dignified death.