Can’t the community manage itself?
No, not really.
It won’t usually welcome newcomers by itself.
It won’t update the technology by itself.
It won’t check for quality of content by itself.
It won’t remove the bad stuff and promote the good stuff by itself (not usually).
It can’t resolve disputes by itself.
It won’t stimulate new discussions by itself when things get quiet.
It won’t make sure most members get a good, quick, response by itself.
etc…
Last year, a former client decided not to replace a departing community manager and rely on volunteers. The rules of this are pretty clear:
If you want to throw hundreds of thousands of dollars down the drain, destroy your community, and lose the trust of your best customers, then don’t replace a departing community manager.
Conversely, if you want to grow your community, gain more support from your best customers, and get the best results from your community, replace the departing community manager with the best person you can get.