A “real source,” because “under a tree” and “inside a house” (Sukkah 9b) certainly do not compel the conclusion that absolutely anything above the sukkah, at any height, invalidates it. Especially since according to the Talmud and the halakhic authorities, being under a tree invalidates only when the tree is what creates the shade. So why compare a balcony not to a tree but to being “inside a house”? Is a balcony 6 stories above the sukkah, and only partially above it, really the same as being inside a house?
Therefore I asked for a real source—because I searched and did not find in the Talmud, the medieval authorities (Rishonim), or the halakhic authorities the statement that “anything above the sukkah at any height invalidates it.” I only found discussions of what counts as being under a tree, and mention that a sukkah inside a house is invalid. So where did the later authorities get the axiom that you wrote?
Thanks.

I’m not familiar with this claim about the Talmud and the halakhic authorities, and as far as I know there is no such claim. When invalid roofing is above valid roofing, it invalidates it regardless of whether it’s a tree, a house, or anything else. No height is mentioned anywhere, so anyone claiming that some particular height is relevant bears the burden of proof.
I also haven’t found a real source that valid sukkah roofing can be made from eucalyptus branches. It simply falls under threshing-floor and winepress waste.

In practice, today, when a large part of the population lives in apartment towers, fulfilling the commandment of sukkah is becoming more and more complicated.
If a person has to take fulfillment of the commandment into account even before the commandment arrives, then the commandment of sukkah actually affects the considerations involved in buying an apartment.
How much should a contemporary person invest in money and effort in order to fulfill this commandment, if he lives or wants to live on a high floor without a sukkah balcony?

(By the way, some say that indirectly, the issue of sukkot and the issue of Sabbath elevators affect housing prices in the Haredi public. Since these things matter to them, they avoid building apartment towers, which raises the price of each apartment.)

You can build a sukkah downstairs. In principle, to fulfill a positive commandment one should spend up to a fifth of one’s assets.

What does a Jew do when he comes to buy his permanent home? He checks on his temporary dwelling first (“Is there a sukkah balcony?”). And I’m torn over whether this teaches us about the greatness of Israel—that even the permanent home is viewed as temporary—or, heaven forbid, the opposite: that they prefer turning the temporary dwelling into a permanent one, so that the sukkah should be as convenient as possible without having to leave their own four cubits.

A person who is committed to Jewish law checks the options for building a sukkah just as he checks whether there is a prayer quorum in the area, and a place to buy kosher food.
That kind of checking shows commitment to Jewish law and taking it seriously, responsibly, and with advance thought.

As for preferring a sukkah balcony over building in the yard and going up and down the stairs:
What’s the problem with that? Just as a person prefers an apartment with air conditioning, or with a storage room and parking, he prefers an apartment that will let him fulfill a commandment without unnecessary running around. What’s wrong with that?

By the way, in Nedarim 49b:
Rabbi Yehuda, when he went to the study hall, would carry a pitcher on his shoulder and say: Great is labor, for it honors its owner. Rabbi Shimon would carry a basket on his shoulder and say: Great is labor, for it honors its owner.
They took these items on their shoulders to the study hall, even though it wasn’t all that dignified, so that they could sit there while learning. And they justify themselves: “Great is labor, for it honors its owner”—even though right now I’m engaged in labor in public, and that involves some embarrassment, in the end the labor will honor me, because I won’t have to keep shifting from foot to foot through the whole lesson.
It seems to me that this is an interesting source for the idea that there is nothing wrong with a person looking after his own comfort, even comfort in the course of fulfilling a commandment. (You don’t really need a source for that, but still.) One could only discuss whether it is worth building the sukkah at some distance from the house in order to gain “reward for the steps,” see Sotah 22 and Bava Metzia 107.

Who said there’s anything wrong with concern for convenience? There was a halakhic question here about how much money one is halakhically required to spend, and that’s what I addressed. I don’t understand the connection to all these comments.