There are two charts on the page in the IATA report that was linked. The chart we have here shows percentages of the whole while the one we don't have shows total numbers.

Looking at the total numbers, premium passenger counts (not sure of the definition, but I'm guessing F/J) sat at about 5.3m/month in early 2008. There was a crash to about 4.0m/month at the bottom of the recession (putting the number at about 75% of the peak level). Numbers rebounded, reaching pre-recession levels in about 2013; current levels are at about 5.6m (although flattening). FWIW, there's a slower flattening of economy travel going on as well.

Doing my best to interpret this, I think a lot of the non-comeback is down to higher load factors in economy on the same planes plus Premium Economy helping to take some demand load off of "actual premium" travel (and, in some cases, allowing Economy itself to get downgraded). The recent slowing in Economy demand is simply load factors getting maxed out.