Of course Blizzard would start designing WoW around twitch.tv and streamers. After all, they are the ones who create free publicity and hype for Blizzard's products and get them record sales... but is this publicity really free?
Nope, it comes at a very steep price for the entire game design of WoW.
Some things, by their nature, are fun to experience firsthand but not very fun to watch. Building a character for slower-paced, more strategic combat is not very fun to watch. DHs zooming around at 200% speed and spamming "insta gib" abilities, however, entertains people.
Raids by their design should not be very fun to watch since they require lots of repetition and wipes to complete. Moreover, they have a lot of individual downtime as different players handle different mechanics. Blizzard's solution? Design raids around twitch.tv. One shot raid wipe personal responsibility run rampant all for the sake for satisfying people who want to spam "LOL POGGERS WHAT A LOSER" in chat when someone inevitably fuck ups and wipes the raid. Why bother designing clever mechanics when bosses can just be series of one shot mechanics being spammed in every direction?
There's more. M+ is without a doubt tailored to please the speedrunning community which is, not surprisingly, also found on twitch.tv.
But worst of all? Twitch.tv breeds rabid fanboy culture and it seeps into the actual game. When was the last time you played a raid or a m+ without some 12 year old kid spamming "POGGERS OMEGALUL YIKES SCUFFED" all run long?
Stop designing the game for idiotic twitch culture and you'll see immediate improvement.