> edit: Downvote brigade... why am I wrong?
Most projections have the worlds population set to start reducing again within a few decades, and then likely stabilise well below the projected peak.
Here is UN's latest revision of their "World Population to 2300" report: https://www.un.org/esa/population/publications/longrange2/Wo...
This report presents a main scenario with a peak of 9.22 billion in 2075, followed by a drop and slow resumption of growth towards 8.97 billion by 2300. It references older UN projections that indicates a range from 7.4 billion to 10.6 billion for 2050 depending on scenario.
This current report assumes 8.9 billion for 2050, and projects annual increases in population sizes down to 0.33% in 2045-2050, vs. 1.22% in 2000-2005
This comes as more and more countries sees rapid decline in birth rates equivalent to those that the developed world has already seen.
Particularly look at pages 6-8 in the report, where you can get an idea what we would expect to see if cancer was "cured": Life expectancy and fertility mirror each other closely. As we live longer, we put off having children, have fewer children, or opt not to have children at all.
Even if we were to cure all forms of cancer, people would continue to die of other causes eventually. At most a cure would lead the population size to level out at a somewhat higher level because we might have children proportionally earlier in a life span so generations would overlap more. But we have to deal with these population numbers without a cure for cancer too, to deal with the bulge around the peak population sizes expected.
While pension-age might be a problem, if people remain healthier longer there's little reason not to increase the pension age, as some countries have already started doing.