While some players are keen to enter the era of 15-man raiding, others loath the roster moves and guild shakeups this would cause. For those that were not around during the 40-man to 25-man transition in The Burning Crusade, many many guilds fell apart over having to make drastic cuts from their rosters. In a series of tweets, Ghostcrawler notes that the transition to 15-man raiding would cause guilds to prune 10 people from a 25-man roster, however I'd contend it'd be much more than that (around 15 people would need to be cut), due to the fact most 25-man guilds carry several more folks than necessary, which wouldn't be needed in a leaner and meaner raiding crew.
However, it appears that a transition to 15-man raiding is essentially off the table. Ghostcrawler tweets: