Earlier quoted context omitted.
If I understand you correctly, you are saying that taboos should not be examined from within the space where they hold effect, because doing so calls into question the whole concept of a taboo and robs all taboos of their usefulness, and that would summon evidence for, or even cause cultural decline? That sounds suspiciously like something a taboo would say that has something to fear from being looked at. ;) I think…
Not should, can't. The assertion I'm referring to is that once something can be rationally discussed it's no longer a taboo, and once a culture has no taboos it has no real vitality or potential for growth left. Nothing there about causing cultural decline, just evidence of it, and thereby limits on the kind of culture that can host the attitude you describe.
This appears to be an entirely impractical definition, to the point that it would not allow a taboo to ever exist in a group of people capable of any sort of rational discussion. Any two moderately curious and independently-minded people could simply destroy any taboo by talking about it. They could even do it on purpose, using this fatal weakness.
Taboos, as in: the actual attitudes and behaviors of people, do not simply disappear the moment they are named or even questioned. I think we have to allow for taboos to be more nuanced, or we will struggle to describe the actual, interesting phenomenon, let alone do anything useful with it.
> once a culture has no taboos it has no real vitality or potential for growth left
That's an extraordinary claim on multiple levels, even when allowing for different ideas of what taboos are and how they work. I already mentioned some questions this raises, which makes it surprising to find the naked claim simply restated, in an even stronger form.
It's the kind of sentence that can sound really deep and powerful in passing, but when you look at it, is really only a huge, gaping question mark in a fancy dress.