That's a very good explanation, but probably not a smart idea from the Stratosian point of view. They could make do without the income from zenite for a while, but if they don't have direct access to the space-durra, astro-aubergine and quadrotrititibarley fields, the Troglytes will have them in a stranglehold in a matter of weeks!
One wonders if the zenite isn't poisoning everything down there, planetwide, making the farmers just as stupid as the miners eventually. Perhaps it wasn't that bad originally, but mining within the past centuries has released so much of the gas that the Stratosians are now correct in their prejudice about the mental capabilities of all Troglytes.
Also, zenite supposedly defeats a botanical plague. What else is it good for? Or is it effective against a wide range of such plagues, and mined for that reason? In any case, we might speculate on the connection between the seemingly infertile conditions down below and the presence of zenite... Perhaps this planet was once lush but was devastated by another botanical plague, leaving only a small community of survivors who had learned to use zenite to fight the malady?
In such a case, though, one would expect a community capable of building floating cities to be capable of a global counterstrike against the plague - and conversely, a society cornered in a small disease-free area could be expected to be incapable of creating a floating city. Unless we're speaking about colonists from outer space, in which case it would make sense for their resources to be concentrated in a small area, and for them to have little interest in protecting the entire planet when they only ever have the manpower to exploit a tiny little corner of it anyway.
Timo Saloniemi