This is a completely incorrect narrative about the removal of flying almost killing WoW. Lack of content in WoD almost killed WoW.
The earliest YouTube videos covering Blizzards Q1 2015 reports and the loss of 2.9 million players are mostly dated May 7, 2015.
Memorial day wasn’t till May 25, 2015.
This means WoW had lost the largest chunk of their players well before the no flight ever or in future xpacs announcement due to lack of content. They still continued to lose subs even after they went back on the no flying. If anything going back on flying was their hail Mary to try and get players back and stop unsubscribing; which it didn’t and it wasn’t till Legion their numbers recovered.