The average uplifting story is only effective on people who are mostly doing OK mentally. Those types of people might see a depressing story and wonder why anyone would ever want to consume something so dark and depressing.

Someone who is depressed will not be able to relate to a typical uplifting story, it will seem unrealistic and naive. A depressing story is meeting them on their level. They will see that someone else understands how they feel and will feel less alone. A depressing story has a chance of affecting a depressed person a giving them hope in a way a regular story does not.

That is my interpretation of the comic.

Lots of fiction collapses under examination.

Yes, over analyzing every little detail and finding flaws in logic is a great way of completely missing the point the author is trying to get across. Your analysis of this comic comes across as borderline satire. There is absolutely no need to examine the physical logic of the comic past the point of “the balloons are metaphors for stories”. There are plenty of ways to analyze and critique the comic, such as examining how well the balloons function as a metaphor, but trying to figure out the internal logic of the world is missing the point completely.