What is mainstream?
According to the dictionary it means, "the principal or dominant course, tendency, or trend." So in other words something that is very popular.
Do you think anime will become mainstream?
I'm not quite sure because in my opinion, most of the newest "fans" of anime today are just people who merely watched a show like Attack on Titan just because it became popular. Unless an even better show that beats the current anime industry now emerges, it probably won't. Looking at it in a different perspective, the average person I know has at least watched one type of anime (If you count pokemon which I believe is a cartoon). Right now we can't tell, if only we had a Mirai Nikki....
Do you want anime to become mainstream?
The selfish answer would be NO. Personally like many other hardcore anime fans, anime is what makes me unique. People who've just watched popular anime shows haven't seen the whole picture. Do you think your an anime fan? Name 20 or more anime's you have watched that don't include, Attack on Titan, and the mainstream shonen (Naruto, One Piece, Bleach, Fairytale and DBZ. You see, many people who are watching it for it's popularity can't do this. But there's another side to this. Shouldn't people be allowed to watch what they want? Not to mention, if the anime industry is booming, they produce more money, which in turn provides better quality anime. But whether it becomes mainstream or not I will still love it forever.