T.S.
The thing about kids is they change their minds. It isn't fair to hold a 9 year old to a choice or agreement she made at 4 or 8 or even earlier this year. Just because you're good at something doesn't mean you like it... it just means you've been doing it for quite a while (5 years is a lot of practice).
If you're concerned about her physical well being (don't want her to become inactive) ask her what other sport she might be interested in... Maybe she would like to dance, do gymnastics, play basketball etc.
6 months from now, she may decides she misses soccer and want to come back, or she may want to do another new activity, but a 9 year old has to allowed to experiment with social and sport activities.
They call it playing sports for reason... it's suposed to be play, not work!
HTH.
T.