That's just rationalization of the existing situation. If it was really so, the people in the past wouldnt have done everything to address the lifespan and increase it as much as they could. Back 100 years ago when people were living to a ripe old age of 40 in the industrial slums there were people who were saying that it was just the 'way of life' and 'how things are', but the society and science did not listen to such luddism and did everything they can to increase the lifespan. Now we have 70 years as the new norm, and there are people who are rationalizing the existing situation as 'the natural'.

Its not. There are species on this planet that live over 300 years. From tortoises to sharks. Which means that there is nothing 'natural' about a higher species having a mere 70 year lifespan. And science and technology will seek to push the human lifespan even further and eventually it will be 100-150 years and there will be people who just rationalize the situation that will exist than as 'the normal' and 'natural'. When the day on which the human lifespan exceeds 300 years and catches the standard of various other species on this planet, there will be people who will rationalize it as 'the normal' and 'the way things should be' too.