Blizzard used to report subscription numbers for WoW, other MMOs don't report them. Yet it was pretty safe to assume WoW has more than the rest combined. Blizzard didn't shame to report when the subscription numbers dropped to a half and even though other MMOs meanwhile are getting shut down, no new content, etc. Blizzard are the ones who got crucified by the "fans" for the lower numbers. So reporting anything below 12 million creates unhealthy atmosphere surrounding their title, why would they keep doing it? Yes, subs will rise for Legion, but they will probably not reach the numbers during Wrath of the Lich King and certainly NOT because Legion is bad (although it may be, we don't know yet). Basically whatever Blizzard says, unless it passes 12 million will be perceived as bad.
Across the board the old style MMOs are seeing withdrawal. How many teenagers you know nowadays who passionately play WoW? Any other similar MMO? Was it like that 7 years ago? Everyone knew World of Warcraft then, today the youth often hasn't even heard of it, not to mention the other WoW wannabe games. Many of the people who used to play day and night in the past have jobs and families today, which leaves little time to play an MMO for 5-6 hours each evening, not to mention 12 or more hours. So yes, if other companies are not "obliged" to announce their subscribers why Blizzard should be? Such information concerns corporate levels and development, not the subscribers.
As long as Blizzard keep doing awesome job, which they do, including the willingness to admit mistakes and improve, which they also do, I am a happy customer.