What does a color grading software offer over a compositor?
The functionality of editing vs color grading vs compositing software is overlapping since every professional software can do a bit of everything. But they are experts in their own domain. Resolve has finer color controls (1), but also some functionality that you won't find in compositing software (2).

Will they merge some day? This does depend on the job roles. If several persons work on a project, each with his/her specialization of editing/grading/compositing, then it's better that each has his/her own program that focuses on just this task and does this best. If job roles blend (as we see with editing &amp; grading), then we see programs emerge that are good at each of the tasks.



(1) Example: A Resolve user can choose between at least three sets of mathematical formulas for the color wheels

(2) Example: Automated shot matching