It was established but it's like the Pokemon craze didn't really blow it up like AoT did.I... find that really does not hold up.
Probably because I'm an oldfag who remembers the days of shopping for anime at Suncoast (mid-to-late 1990s), but the boom from "niche" to "mainstream" actually began with Pokemon. The switch from it being called "Japanimation" to being called "Anime" happened almost right after the Pokemon boom. Then suddenly we started getting more shows, DVDs became easier to find, and even Shonen Jump started publishing in the US.
Anime was already pretty established before Attack on Titan was even a thing.
I think this has long been one of the reasons people gravitate to anime. Even in my time I recall the deal was people just weren't satisfied with what westerners were doing.
When Pokemon was incredibly popular it was treated as a kid's show but when AoT came out it seemed like everyone was talking about it: Like I saw people comparing it to Game of Thrones that weren't even weebs.
So that's my perspective: After that it felt like anime just progressively got more and more mainstream.
Still I don't know exactly when it went from Niche to Popular to Mainstream, just know it did, and all I have is just my scapegoat to blame for everything I don't like.