It obviously has 'any' validity. If an instance of 'ancient wisdom' killed off or weakened the followers enough, it wouldn't be around. Also, said thing has been optimized for a lot of time by a lot of people, and the version we receive probably isn't the best, but still one of the better versions.

While some will weaken the people a bit and stick around for sounding good, they generally are just ideas that worked well enough. The best argument for 'ancient wisdom' is that you can actually just check how it has effected the people using it. If it has good effects on them, it is likely a good idea. If it has negative effects, it is probably bad.

'Ancient wisdom' also includes a lot of ideas we really don't think of that way. Including math, science, language, etc. We start calling it things like 'ancient wisdom' (or tradition, or culture) if only certain traditions use it, which would mean it was less successful at convincing people, and less likely to be a truly good idea, but a lot of it will still be very good ideas.

By default, you should probably think that the reasons given are often wrong, but that the idea itself is in some way useful. (This can just be 'socially useful' though.) 'Alternative medicine' includes a lot of things that kind of work for specific problems, but people didn't figure out how to use in an incontrovertible manner. Some alternative medicines don't work for any likely problem, some are more likely to poison than help, but in general they solve real problems. In many cases, 'ancient wisdom' medicine is normal medicine. They had a lot of bad ideas over the millenia medically, but many aso clearly worked. 'Religion' includes a lot of things that are shown scientifically to improve the health, happiness, and wellbeing of adherents, but some strains of religion make them do crazy / evil things. You can't really make a blatant statement by the category.

I'd read Nassim Nicholas Taleb on the Lindy effect for the strongest defense of this proposition. Basically all ideas and culture are constantly fighting in a market of culture. For every Jhana there are 1000 different spiritual concepts, which try to occupy the same niche. There has to be something to Jhana's that leads to it still being done today, while the rest became history. That something does not have to necessarily mean that the idea is true, for example meditation in general is known to be very good for you, so if Jhana's work as the carrot on the stick to get people to meditate, then the idea would also stick around, as people start praising Jhana's due to the benefits they got from meditation. But every idea that is old needs to have some sort of payload, something that helped it survive on the market of ideas and culture for millenia.

I would tend to give particular credence to any practice which pre-dates the printing press.

The reason is fairly straightforward. Spreading ideas was significantly more expensive, and often could only occur to the extent that holding the ideas made the carriers better adapted to their environments.

As the cost to spread an idea has become cheaper, humans can unfortunately afford to spread a great deal more pleasant (feel free to substitute reward hacking for pleasant) junk.

That doesn't mean failing to examine the ideas critically, but there are more than a few ideas that I once doubted the wisdom of, which made a great deal more sense from this perspective.

As for the particular practice of meditation that you reference, I tend to view spiritual practices as somewhat difficult to analyze for this purpose, as the entire structure of the religion was what was transmitted, not only the particularly adaptive information. To use DNA as an analogy, it's difficult to tell, which portions are of particularly high utility, analogous to the A, C, G, and T in DNA, and which serve as the sugar-phosphate backbone. Potentially useful in maintenance of the structure as a whole, but perhaps not of particular use when translated outside that context.

Which portions of Buddhism are which, I couldn't tell you, I lack practice in the meditation methods mentioned, and lack deeper familiarity with the relevant social and historical context.

Age and popularity of an idea or practice have some predictive power as to how useful it has been.  Old and surviving is some evidence.  Popular is some evidence.  Old and NOT popular is conflicting evidence - it's useful (or at least not very harmful) to some, perhaps limited by context or covariant factors that don't apply elsewhere.  

Whether your interpretation of a practice will get benefits for you should probably be determined by more specific analysis than "it worked for a small set of people in a very different environment, and never caught on universally".

It seems plausible to me that there is a sort of selection process in which people are creating ostensible-wisdom all the time, but only some of that wisdom gets passed along to the next generation, and the next, and so forth, while a lot of it gets discarded. If some example of wisdom is indeed ancient, then you can by virtue of that have at least some evidence that it has passed through this selection process.

To what extent this selection process selects for wisdom that actually earns that designation I'll leave as an exercise for the reader.

Some stuff just works but for reasons unknow to the practioner. Trail and error is a very powerful tool if used over over many generations to "solve" a particular problem. But that do not mean anyone know WHY it works.