If only it was as easy as "someone make a new Twitch". Twitch is HUGE and had a lot of problems growing to the size it is now. Scaling that big isn't something any old company can do. There's also the whole social aspect. Who's going to stream on a competitor site and have 5 viewers when they can use Twitch and have 5,000? Sometimes the MySpaces die and are replaced by Facebooks, but it doesn't happen often and not unless the new thing is considerably better than the old thing.

There already are alternatives to Twitch. Main one I know of is http://www.hitbox.tv/