@TheDailyCarbunkle (I like your nickname), the study you linked to looks ahead to the year 2100. I think their forecasts are off. At the most basic level, this is why:
In the years 2000 - 2021, in the UK,
13.1 million people died
16.4 million people were born.
The net population increased by 3.3 million (crude figures).
Now those babies will be at least 80 by 2021, and probably half of them will be dead. Maybe more, as life expectancy here is reducing.
The question of whether there will be enough younger adults to support the elderly survivors is unknown; those people haven't been born yet.
Studies like this tend to assume birth rates will continue to decline. But there's no reason why they would - people will still want children, and the past couple of years have shown an uptick. The 16 million people born in the last 21 years will have had children, be grandparents & great-grandparents.
Declining birth rates aren't due to any pervasive health problem, it's purely socio-economic. If this thread's doom predictions come true, perhaps nobody will be wanting children - but if we're adopting general doom all around, it hardly matters as we're all going to die prematurely! If life muddles along, people will find ways to have kids and keep them healthy enough to have some of their own.
I just think it's an unreasonable assumption. I have a mean suspicion that the big problem of too many old people is being over-egged because our elderly are increasingly seen as a useless waste of money, nudging public opinion towards euthanasia (hello, Canada).