The true fact is that Rebirth flopping so badly that Square Enix has still to this day never released any official sales celebration material was the final nail in /ffg/'s coffin.
/ffg/ had previously been just as lively as was normal, replete with constant, gleeful and daily shitposting by oneguy. It wasn't until Rebirth flopped that he became upset. XVI had nothing to do with it, since all the XIV shitposts simply turned into XVI shitposts, and there truly was no actual change in /ffg/'s structure from its release.