I've let two sponsors go and been let go by one sponsor.
The two I let go: The first turned out to be nothing more than a coffee sponsor-we'd meet for coffee and really talk about anything except the solution... I was desperate, needed the solution I saw others living in but it became evident that he wasn't going to be taking me through the book and the program of recovery contained in it. I had to find someone that was willing to do that with me. I was nervous at first, felt like I was breaking up with him, but I had to do it. As a side note, he died drunk and homeless about three years later....
The second sponsor I had let me go when I failed to follow his directions, I hate the word fire in these instances because it denotes some kind of power dynamic in the relationship, a dynamic that is absent if the relationship is healthy. Anyways, he was busy and there were other people in the community that were willing and desperate who needed him as well and he asked me to find another sponsor with the understanding that if I was willing to do the work, then he would work with me.
The third sponsor I had for eight years. He was great, took me through the book and steps, let me into his life, his home, his relationships, to show me precisely how he had recovered and how this solution looks in his daily life.
When I moved away (to a new state) I found that I needed more, I hit a bottom in sobriety when I was basically sponsoring myself and found that I needed a new teacher so I could revisit the work and again have my life changed. My current sponsor was put into my life then and he has been amazing. Taking me back through the book, through the work, etc.
I definitely think it is possible for people to outgrow the sponsorship they have, there is nothing wrong with needing new insight etc.
Be wary of celebrity sponsorship though, I'm always leery of "sponsorship lines" that trace their lineage back to person x. Seems to make deities out of people.... Putting that much reliance upon the words of a human, in my experience, has been problematic. For me it was necessary to find a real drunk who had recovered from this thing and seemed to embody the spirituality that I wanted.