It could also be the other way round. People might be discouraged to run
    against the long-time chairs.


**** Exactly, that's was my point, it looks that I was not able to find the best wording. I've seen that in other organizations, not just here.

1) You ask for volunteers to replace "existing co-chair a"
2) There are 1-2 volunteers
3) "existing co-chair a" say, I will also volunteer to continue
4) There are comments like "existing co-chair a" has been there for long time, he knows the job ...
5) the 1-2 volunteers then drop-out.