Long term abuse to the point that "users don't ever feel the same without it" is quite concerning and very problematic, but once or twice seems fine, despite the absence of "a century long twin study".
Because this is such an incredible oversimplification of what goes on in the brain. SSRIs also dramatically increase the overall serotonin flux, to the point where if you quit suddenly, the brain is left in a deficit, the effects of which can be quite unpleasant. And contrary to the "serotonin is happy chemical, more serotonin = more better" myth, the net result of all this serotonin is that 5-HT receptors actually downregulate. Ultimately, the result of all these changes is an increase in neuroplasticity.
Blasting 5-HT receptors (eg by LSD, psilocin, etc, and to an extent MDMA) is also shown to increase neuroplasticity.
Neurotoxicity and psychopharmacology are extremely complicated. It's beyond reductionist to say "drug floods brain ergo bad".
Bizarre we have gotten to the point where the default is "take powerful drug because no scientist has proven not to in a century long twin study!!"