There are some countries with an upside down population pyramid and it isn't the end of the world. Certainly less apocalyptic than most climate change scenarios.
Well, it doesn't scale for the future tax vs. elder social benefits. You can't just sweep the consequences of extreme population demographic shifts under a rug. Look at China with its disastrous One-child policy where there are millions of unhappy, dissatisfied men who can't marry. That's a recipe for social unrest and suicide attacks.
Look at Japan, there things got more stable with the aging of the population. China isn't really an example for most of the world because CCP experiments screwed up more than just their age pyramid.
Also, taxes and pensions are very small problems compared to uninhabitable coastal regions, mass migrations and food shortages that climate change predictions show us.
Taxes and pensions are pressing, immediate problems within the next 50 years. What you're talking about is a much bigger problem trend, but it will take longer to occur, say 80-120 years.
Age pyramid? That's not the biggest issue in China's demographics: it's the sex ratio. Talking vague FUD about China isn't helpful and doesn't contribute to this discussion.
Mass migrations and food shortages are happening now, but not as bad as they will be. This is due to both climate and US Monroe doctrine meddling (crime caused by the War on Drugs and installation of right-wing leaders with death squads).