Please point me to said statistics by Blizzard. I'd love to take a closer look.
I've checked the clearing numbers for BRF and TOT on the highest difficulty today and player numbers dropped for those by 75%—while subs only dropped by 22% in the respective timeframe. Most certainly doesn't look like a fabricated problem to me.
Darwinism has nothing to do with not being able or willing to adapt to the new size. Most people simply don't enjoy a bigger raid size, be it for logistical reasons from the leaders'/officers' perspective or social reasons from the member's perspective. In MOP, 90% of the players were raiding 10m on the highest difficulty and only 10% 25m. That clearly shows the preference of the players imo.