That question sounds stupid. Twitch is growing by the day. And they recently received 15 million dollars from an investment firm. What is there to worry about?
First, Youtube's size. Would a streamer rather be featured on Youtube's front page or Twitch's? Is a stream more likely to go viral on Youtube or Twitch? Which do you think is more important to TotalBiscuit and Husky: their Twitch subscriptions, or their Youtube subscriptions? Youtube simply provides a bigger audience than Twitch.
Second, Youtube's VODs. Despite Twitch's repeated promises that they're working on making their VODs persistent and searchable, no significant progress has been made. That streamers like Day9 and Destiny move content from their Twitch streams to their Youtube channels should be a big red flag. If Youtube can synergize streaming and VODs, why would anyone choose to just stream on Twtich?
Youtube hasn't made any inroads yet. FXOpen's experiment with Youtube Live Streaming seems to have failed, and Twitch has undoubtedly developed personal and contractual relationship in the community which it can leverage. Still, if I were DJWheat, 2GD, or Gunrun, I'd be worried.