Twice's J-line isn't the only reason they are so huge in Japan

The J-line are a reason yes, but the aren't the only reason. The reason Twice are so huge and have stayed to huge is because they put in the work. Japan is just as important as Korea to Twice. Twice release just as much if not more music in Japan as the do in Korea. They also do Japan exclusive season greetings, fanmeetings, reality shows, merch, and basically everything the do in Korea they also do in Japan. Members, even they non-Japanese ones, model for or endorse Japanese brands as well. Twice are essentially both a K-pop and J-pop group at this point. I can't think of another girl group that has treated any other country besides Korea with this much importance. And if any girl group wants to surpass Twice in Japan they are going to have to put in just as much work as Twice have.