It's a bit of both. The withdrawal effect from decreasing a dosage, or going off cold turkey, won't "make" depression return necessarily, but it can open up some vulnerability to it due to the somewhat shaky neurochemical state.

Also, there's usually a bit of a blowback effect from decreasing some of the calming effects that were previously present (from the higher serotonin levels), which typically leads to heightened anxiety and agitation. Mostly in the short-term, but it can be tricky to shake off the fearful state even after a person's system starts to rebalance itself.

You're right about one thing though, the exact nature of the "supposed" therapeutic effects from SSRI's are... basically something that can't realistically be quantified or measured. There's some hints at what some of those neurological effects are, but they can't trace it with any kind of precision or accuracy. T

here's not any specific chemicals or by-products they can trace, because it's not specifically the serotonin that's supposedly the therapeutic effect. It's the by-products produced from ramping up that serotonin uptake. And those effects will not really correlate with how much of the active ingredient is measured in the bloodstream.

The serotonin CAN however have a direct therapeutic effect on anxiety related symptoms however. But for the purposes of developing and marketing drugs as antidepessants? The serotonin itself was never intended to be the main fix, though they did end up marketing it that way, likely because it was easier to try and explain in laymen terms.