And yet, we can see which economies are vibrant and which are stagnant.
Economies and societies are not one dimensional though. They can have lots of positive developments that keep them vibrant, and they can simultaneously face their own unique issues such falling birth rates, ageing populations etc.
In this particular case, what low marriage and birth rates lead to is that they makes your demographic distribution heavily tilted towards older generations:
Meaning more of your current population is retired, more reliant on the state for pensions, social care, government funded healthcare, meanwhile their contributions to taxes are limited due to being retired. I've read in depth what an ageing society means for the UK's fiscal stability, long term trend rate of growth, and the sustainability of government debt. To combat this governments look to promote capital intensive methods of production that cuts the need for labour, but ultimately they all invariably end up relying on high levels of immigration even though that choice is unpopular among natives. Some governments also come up with some really good policies like mandatory paternity and maternity leave, free or subsidised child care, etc.
But the more lopsided that chart gets, the fewer and fewer working age people you have to support a burgeoning amount of older folks.
You can be like Japan and have an ageing population, and still be fairly vibrant. You can have vibrant areas where people emigrate to like Tokyo and Osaka, but you will also see previously vibrant areas fall in to depopulation and decay. You can even use the lack of people to push for more and more digitisation, automation, and make strides in capital usage. But these are not virtues of in and of themselves, they are a response to an undesirable issue.
There is little doubt that deflation (called secular stagnation by Larry Summers), ageing populations, a shrinking pool of working age people to pay for increasing upkeeps of an older generation - are all related trends, and they are all considered undesirable.
I'm not sure about the US, but in the UK and the rest of Europe, this is seen as a major issue. You can refer for example to the UK OBR's
Fiscal Sustainability Reports, and see for yourself in what terms they address this issue, and the consequences they see.