You know what? I reckon we probably won't run out of people any time soon.
No, indeed not, but we're already struggling to cope with the ageing of that huge population spurt. We're not, as you say, going to run out of people, but none of those people are getting any younger, and most of them are going to live to be far older than ever used to be the case.
Not all that long ago, people taking a lifestyle choice not to have children would've been unheard of. Back when there was no state pension, no welfare state, no NHS; no organised society, in effect, families worked on a simple protocol of one generation caring for the next whilst they are too young to care for themselves, and then that generation in turn caring for the previous generation when it's their
turn to be too old to care for themselves. Anyone deciding not to have children would've been pretty much stuffed when they became too old to work, because there wouldn't have been anyone else around to pick up the care, unless they'd been fortunate enough to save enough to employ servants.
Of course, for a long time this was also pretty academic, as on average people only survived maybe 5 years or so in retirement anyway. Move forward to the 21st century, however, and we're living 10, 20, even 30 years in retirement. My maternal Grandfather retired at the age of 60, and didn't die until he was 97! If he'd made it to the ton, he would've been retired for as long as he'd worked.
Take another look at your graph, and look at when the peak started to build, and how quickly it did so. Couple that with huge increases in life expectancy, which have not been matched with any real increase in work
life expectancy, and regardless of how many people there currently are in this country, there aren't going to be enough people of working age in 10 - 20 years to cope with the retired population.
When we really reach breaking point, what is going to happen? Unless massive changes are made very quickly - which will be hugely unpopular, so knowing our spineless politicians means it won't happen - then there will come a time when the state simply cannot tax enough to support the elderly. At that point, support systems will break down, and there's a good chance we'll find ourselves reverting to that point where each family supported their own, because we've lost the notion of society being able to do it on a macro-economic scale.
So, who is making the lifestyle choice?
Is it the people who choose to have children, thus following in the pattern which drives every species on the face of the planet at their most fundamental level, or is it the people who've decided not to have children, who, for the last few decades only, have been able to do this as the state has used the burden of taxation to remove the burden of care?
Time will tell, but personally I'm not convinced the burden of taxation is going to be bearable for very much longer.