Yes, a big one, as the article explains: if you have a massive number of old people living on pensions / versions of social security, but not a lot of younger people working to produce goods and services, those old people are not going to be able to survive at the standards of living they'd like and b) over-taxed young people are going to be incentived to not work or leave.

These are not good things, especially because they have recursive aspects, as discussed.

Children are a Ponzi scheme - you have lots of children to support you in old age (either directly in 3rd world or via social security in the 1st) This leads to over population - but screw them, you will be dead.

It's rather like some countries attitude to consuming natural resources, if we don't burn the oil first somebody else will

That's a problem that will solve itself in a maximum of about 70 years.

No, it won't. If you maintain the same sub-replacement fertility rate, you'll end up with a stable inverted population pyramid, albeit a constantly shrinking one. Every generation will have too few workers to support too many retirees.

its literally a catch 22 situation, its a matter of what perspective (regarding the time factor) one is looking at.

Over population will cause pressure on the sustainability of resources on the planet, not to mention inefficient use of resources and resulting pollution.

Under population in a society will put other pressures on the sustainability of a society - more of the economic type and on a longer time frame. The pressures would be more about having a critical enough market size that can support enough jobs and hence a decent enough quality of life.