I think it was more a case of Casemiro wanting a new challenge and the club respecting his wishes. Both Modric and Kroos are significantly older than him and are still playing, I’m sure he could’ve continued for a few more years as well if he had wanted to.
Musiala is better at close range dribbling in tight spaces. If he starts capitalising on that skill with a better final ball/ shooting I think he might have a higher ceiling.