First, thank you for your question. To answer, as someone who has been watching this thread closely and crunching numbers, what we are trying to convey is while what you are seeing in-game with people being less present, particularly at endgame, is true, the exacerbated nature of the statement "75% of the playerbase" is not supported by the data when using one-to-one comparative methodology. Meaning, comparing the peak numbers at launch to peak numbers in the last month for Steam activity does show a -73% change. But peak to peak comparison only captures a singular moment in time, a single minute, when the number was reached which is not representative of an overall trend of activity because no one is playing all day every day (a statistically small amount keep the game running 24/7). Therefore, the more statistically meaningful number is the average which is calculated over the entire month, displaying a -61% change. A significant change, no doubt, but as Striker mentioned not that different from other post-expansion launch periods leading into the x.2 patch.
To address the other point, the largest factor why we believe the Steam numbers are somewhat questionable is because we don't have the data on what the regional distribution of those numbers is, and by doing some light looking around, it is likely that the JP servers are being underrepresented in that data. Combined with Lucky Bancho's raw data by server that reflects the largest fluctuations in server population are in NA and EU servers (also noted by Lucky Bancho as normal), we are reasonably alleging that the Steam data could be skewed to be largely representative of mostly NA and EU trends without the equalizing effect of JP. This leads us to the estimated conclusion that these numbers may represent the significant shift in the NA and EU player population, which was expected due to multiple factors both inside and outside the game, it is unclear that it can be used to claim that it represents the entire player base due to the lack of evidence that JP is adequately represented.