That’s true for Dothraki, but Klingon is not a professionally created language. It’s largely the result of the Star Trek fan base building upon a kernel of vocabulary and grammar created by the movie producers.
that you will be very lucky indeed if these non-natural (=artificially created) languages have more than a couple of hundred root words and nary an exception to a noun declension or verb conjugation amongst ‘em
That’s probably true for the vast majority, which are languages which no one has ever heard of. But the ones we’re talking about, Klingon and Dothraki for instance, don’t fall into this category.
a crazy pointless youthful hobby
I’m not saying that such exercises will solve global warming or end world hunger, but they’re not pointless. (And not always youthful, either.) Inventing such languages is a good way to explore the complexity of language, to understand how the different aspects interact with one another (e.g., how phonetics influences etymology). It’s not a replacement for the formal, traditional study of natural languages, but it’s a useful adjunct.