To be honest I can't really explain how the heck Utada managed to crack the top 10 in the US. I think timing could have something to do with it. There isn't any BIG release out right now, so there isn't too much competition going on in the charts. I am almost positive it will sink quickly.
Utada is the most well known Japanese singer here, Kyary is probably the second. She has a small but highly devoted fanbase so what I think happened was even though she has a small cult following, a lot of the fans actually bought the album, allowing it to chart high briefly. It actually happens a lot here. Some lesser known album will randomly appear and then in a day or too disappear because all the die hard fans bought it day one and then it stops selling because the mainstream doesn't care.
I won't even say anything about the KH/Utada fans here, simple and clean and passion on repeat for the rest of their life.
But I would say there is a strong chance she will return for the new game. The demand here is very strong. I mean she even did Sakura Nagashi while on hiatus just to honor the Neon Genesis film series.