I find it interesting to your point how even the way we make sense of something like this brings its own contextual baggage. Even "complexity" as a word can be good ("more powerful, developed") or bad ("more fragile, inefficient, and complicated") depending who you ask in what context.
Take this study: it seems to be exploring a hypothesis that societies that have more adult learners of a language tend to simplify that language to make it easier to learn. It seems quite interesting, and it gives us perspective on something we generally take for granted every day. It also seems to cast doubt on that original hypothesis.
But if this same study was called a measure of grammatical complicatedness, with the "more complex" languages instead called more complicated, and the "less complex" languages instead labelled more efficient or streamlined, we'd likely interpret the information differently.
So taking the summarised posted article. A quick glance reads to me as "some languages are more complex than others, and the hypothesis is that languages tend to reduce their complexity to allow non-native speakers to learn it". This to me, in the Western Enlightenment tradition of the march to progress where "the more something has been built upon and developed, and the more function and capability it has, the better it is" doesn't necessarily read as a positive thing at face value. But we could just as easily read it as language adapting and knocking down barriers to allow people to create connections and feel welcome in ways they were excluded from previously. Or as supporting international trade and being more efficient. Or as you've highlighted that perhaps they've altogether misunderstood the complexity of other languages by interpreting them in terms informed from their understanding of the complexity of their own native languages.
Conservatives in the Burke/Tocqueville tradition (as a quick tangent, a tradition that in my view at its best is like programming aka Don't Reinvent the Wheel, Don't Rewrite from Scratch, Refactor Respectfully after Learning Why it Is How it Is, but at its worst and sadly most common comes across far more as a romanticised and overglorified appeal to return to a time of greater privilege, exploitation, and ill-gotten misused power) would probably bring in another angle arguing that if the hypothesis were true it demonstrates a decay of civilisation: that there used to be these "great" languages that we're losing/lost.
I think you make the great point that there's so much subjectivity in this: that different languages can be useful in different ways without resorting to value comparisons.
We could apply the same lenses to memes and/or emojis: some argue they're a sign everyone is getting dumber and less articulate in the Idiocracy tradition, while others argue they're exciting and creative new culture, or an expression of increased global connection, or something else altogether.
Reflecting on English itself, we historically have seen a bit of both/and to the original hypothesis: a "simpler" more utility style of English that can selectively borrow on more "complex" words. The simpler English appears to have been used as much by locals who were not educated in the language of their new rulers as for any trade/migration reason, and accordingly has been heavily influenced by the older Saxon roots (an interesting outcome given oppressors, including the English, have at other times inhumanely erased the language of those they've conquered), contrasted with the French influence of upper-class conquering Normans whose words are usually considered in English more "complex" and perceived as more refined/intellectual by virtue of them being the language of power. Managementese is probably a modern incarnation of the same idea.
Looking at this differently and taking the "language can be useful in different ways" point, English seems generally still more likely to lean on its Germanic roots in describing practical matters, and French roots in describing aesthetic ones. Indeed its ability to borrow on different traditions is in my view one of its great strengths.
I'd probably argue the more language the merrier as long as we can all express what we want to express and understand each other, but there is probably a risk to entrenching stratification, whether it be of class or race or some other lens (i.e. everyone can speak a common English everyone understands, but the way in which it is spoken and the words selected reveal elements that can have social connotations, and those are often used by those in power as tools of status and judgement). American English seems to have some commonly explored lenses in media in which different styles of speech are heavily culturally and historically influenced and clearly reflect group membership (e.g. contrast Valley Girl English with African-American Vernacular English). We even see a similar idea across generations with slang. And then on the other hand there's the idea of "hidden transcripts" where an oppressed group inverts this model and develops and uses language and/or action (e.g. theatre, music) in a way to communicate in code without tipping off the oppressors (https://en.wikipedia.org/wiki/James_C._Scott#Domination_and_...).
Fascinating space given how much we take it for granted!