Successful? Yes. Popular? Absolutely. Legends? Not quite.
I'm a big fan of Twice, but it's too early to be calling them that. I want them to get on the same level of success and legendary status as SNSD. I want them to be as big as SNSD--I love them both and I think it would be cool if Twice inherited the legacy that SNSD has created and will leave behind. But they've only been around for one year--that's not long enough to see how big of an impact they'll have on the industry. Look at 2NE1 and the Wonder Girls--they all debuted around same time/generation as SNSD and yet for the most part, only SNSD has the status of being called legendary. Why? Because they've managed to stay relevant and active for almost a decade now. No offense to any fans of 2NE1 and Wonder Girls, but SNSD has had some really negative scandals (just look at Tiffany and the infamous 9/30/14 JessicaGate) and has gone to America to debut, but they've still managed to keep their popularity. Obviously it's different since JYP screwed the Wonder Girls over and YG just likes keeping his dungeon full, but SNSD has basically done the same thing and still have come out on top. Look how much a year can change a group's success--2014 was awesome for 2NE1, but with Bom's scandal, Minzy leaving the group, and CL trying to break into America, they've lost a lot of the traction and steam that kept them on top. But Jessica left in 2014 and SNSD still came back with strength. Anything could happen to Twice like what happened to both 2NE1 and SNSD in 2014--the only question is whether they can rebound or not. If they can rebound from multiple scandals and come out on top like that, I will consider them legendary.
When Twice has been around for ten years and managed to become one of the top girl groups to sell the most on their world tours, become the most listened-to girl group for nearly a decade even in the years when they don't release anything, and inspires the next generation of girl groups after them, then I will be more inclined to agree that Twice are legends. For now, they are nothing more than the most popular/trending girl group in Korea at the moment--and that title can change anywhere, anytime.