FPH is gone because it was banned, and it was banned at a time when it was very active, provoked a massive backlash in the immediate term, and the ban appears in the long run to have had the effect of stopping the activity.

I guess your argument for the decline in activity not coming from the ban, but scaling up and down in relation with the prominence of fat acceptance, would have to be that the ban coincided with fat acceptance losing steam around the same time, which I don't think to be the case. (It's also a way of talking that seems to equivocate on whether the behavior was acceptable, since it can be explained as a response to something else, and I think its unacceptability is important enough that it needs to remain clear in any conversation about it.)

I don't have an obvious way of measuring the relationship one way or other. But the face value version of events, which is just that the ban worked, full stop, requires fewer coincidences.