Very good answer. In the case of SSRIs, the brain's reaction, its developing tolerance to the dosage, is exactly how the beneficial effects for anxiety and depression occur. And this physical adaptation takes about two weeks. But it happens by initially introducing something that's pretty uncomfortable, essentially making the problem worse (increasing anxiety, agitation and nervousness) until the brain has a chance to adapt (in this case by making serotonin receptors less sensitive, recessing them into the cell wall or whatever specifically they do).
(And the other beneficial effects - feeling happier and more confident - come over time, weeks, months, years, as a result of being less sensitive.)
And you're right, it's a good question what happens after it's withdrawn. And the answer is usually that eventually the depression comes back.
You're also spot on about the oversimplification and denial and misunderstanding of how they work, and that's lead to all kinds of problems, like all the confusion over "if they make you happier, how can they increase suicide?" question, which is no mystery at all, and even expected, when you understand how they work.