The validation for many/ most things in the modern world (post-industrial revolution) is... the modern world itself; and its technological capability, material abundance, comfort and convenience.
But ultimately - the success, the 'truth' of modernity is the number of people it supports. The validity for modernity is the fact that we have seven billion people in 2019 whereas 200 year before there was one million.
Or, on a smaller and more immediate, observable, scale: for most of its history the population of England did not exceed 4 million (mostly it was a lot less) but from about 1800 it began to increase to about 50 million natives in about 1970. This increase was very obvious, and was obviously caused by big families; those with three or more surviving children.
The ability to support a much larger population was a sign of success; the much larger population was itself an agent of success - enabling the English to spread to may places around the world.
Population growth is, indeed, the most effective validation of modernity. It is the most objective measure that says: the modern world works.
So the first-glance validation, at least up to around 1970, is that modernity works.
But it was noticed even during the later 1800s that those who were most successful in modernity (highest status, riches, most educated etc) were usually the least fertile, had the fewest surviving children - and this especially applied to women. The most successful were indeed subfertile - had fewer than two children per woman.
Subfertility then extended, over the next century, to include entire national populations, then the entire Western world.
If early population growth validated early modernity; then later population decline invalidates later modernity.
The conclusion is that modernity used-to 'work'; but now does not work.
Is this a problem? It is indeed a problem, because people have changed the rules, moved the goalposts - and are now defining the 'success' of modernity as population reduction, instead of population increase!
And matters are worse than this, because the modern world uses its technological capability and material abundance to increase the population in the non- or -less modern parts of the world. The modern world population declines, but transfers modernity to the not-modern world and expand it.
Modernity is not only declining, but hastening its own decline.
Yest surely this a a soluble problem. The power of modernity to solve problems is legendary... But this has not happened - population decline of modern populations has indeed spread to all developed countries without exception. That is the past.
And is not happening in the present; mostly because the problem of population decline is not acknowledged, is denied: indeed - the problem is redefined as success!
So, by the major and most objective criterion with which modernity judges its own success, its own superiority as a world view; modernity has failed; and modernity has been for several generations objectively a failure.
And if modernity is a failure, then the ideas, ideology, assumptions that fuelled modernity are a failure - simply by applying the exact same criteria by which they used to be judged a success.
Modern modernity is objectively a failure. That modern modernity is a dishonest failure ensures that the probems will not even be addressed, never mind solved.