I think this topic can be broken down in 2 questions:
If the injury doesn't heal anymore and Kim is forced to switch to a one-handed backhand:
1) What rank could she achieve with that one-handed backhand?
2) What's better for her: Retiring at age 21, or playing for that highest possible rank you just mentioned in question 1.
1) About 20th-30th place, with a solid slice backhand should be the highest possible rank for her.
2) As a tennis fan i prefer to see her staying active, and playing for that 20th-30th place rank. Just for herself, i think it might be better to retire since 20th-30th place is really bad compared to her current achievements, doesn't help her career, and might not be worth the effort.