It'll probably be after the next raid tier, in the next expansion, has been out for a month or two and most of the top guilds have gotten their Cutting Edge...
This will get streamers happy as their audience gets bored of them farming the same raid for the 8th week in a row. Streamers are free marketing for Blizz. Keep streamers happy with content and that's free advertisement.
At least from a business perspective, if I were in charge, that's how I would play it.
They could also use it to derail another games launch or expansion as Blizzard is known to do.