Another theory for the hell of it-- maybe the Piranha language once had recursion, but eventually got rid of it because the concept wasn't needed, kind of like a language shedding noun declension endings because the prepositions alone were enough.

I would consider this a bombshell if the linguists were saying "wow, these people are using language in a way we've never seen with structures we've never seen" rather than saying "these people don't use particular language structures that we believed all languages did in fact use." As it is I'm not too impressed because lack of use of a structure in the brain does not prove lack of presence in the brain (along the lines of your theory). I believe the brain is quite malleable, but with some very, very hard constraints.

you are the media you consume.