I've seen the problems of loss of support for applications both when lead developers leave and when bought in systems are no longer supported by the lead developers so I'm not sure that either way protects you from losing general support.

The way I would tend to protect against that is try to target the technologies in use to those for which there is a market for developers and yes have a rolling apprenticeship in developer section to ensure new blood regularly comes through and is taught up on existing systems. If the management isn't good enough to manage that then the management ain't good enough.