If she left for Frozen, I can't say that it's a good choice. In Carousel, she was a serious award contender and a Tony can do wonders for an already pretty healthy acting career. The actual longevity of the show doesn't matter, so long as the show doesn't close until that actor decides to leave. Frozen may be a smash, but what does it matter if she only plans on staying with the show for a year. Carousel will do well. It might not be a massive Hamilton/Bette Midler level of success, but it's a beloved show that will do well at the box office and will probably do well at the awards too. I can't say that Fozen will do too well with critics or awards. Therefore, it probably wouldn't be a fantastic career move.
Also, Frozen sucks. No disrespect to the performers, they're doing their best, but the show is garbage. The film was garbage too. It's just a very weak score and story, especially compared with the likes of Beauty and the Beast, The Lion King, and even Aladdin.