I've been curious about the extent to which people commonly use some of these long theoretically possible Turkish words. I was curious about that for German, too, before learning it; my impression since then is that German compound nouns with more than three or so base nouns are rare in practice (maybe outside of a few technical contexts, like names of legislation). Germans love to give examples about a Danube steamship company captain's hat or a rhubarb-loving barbarian's bar and so on, but they probably wouldn't actually use such words spontaneously. So, how does word length work out in day-to-day use of Turkish?

Anecdotally as a second language speaker of Turkish, day to day Turkish doesn't​ use words that long. That's mostly just one of several very similar phrases used to point out how long and creative Turkish suffixes can get. However, many words one uses in day to day conversation have two or three suffixes tacked on. I'd say most words in a sentence have at least one suffix. Formal written Turkish you might find in a newspaper article discussing the latest political developments uses words with even more abstract suffixes. Such formal writing also uses long winded sentences which would be considered run-on in English and could occasionally be translated into a whole paragraph. It's a fascinating language!

You're right, such monstrosities are quite rare in regular language. Also, technically, the "mısınız" part at the end should be written separately so even this example is two words.

Counting the syllables I get something like

Turkish: 16 English: 26

Given the inherent loss in efficiency in trying to accurately translate very specific phrases out of context, that doesn't seem like such a big difference. (And I'm sure you could further condense the translation maybe like "Are you one of them we failed to get to take better care of themselves?" at 18.)

I think the OP's assertion is generally assumed to hold for natural languages.