One thing that is forgotten in this debate is that CC are able to apply for other roles via the internal system like any other staff member.
I know of many who have, and some of those ended up in senior management roles, including Joy Hordern who went on to become Head of CC in the late 1990's.
There is no neccesity to stay as CC throughout your BA career, but thousands do, because they enjoy the lifestyle and the generous allowances.