That's a good reason to have shared responsibility of code. There is no one owner of a codebase, an entire team should own the code. An entire team leaving at once seems highly unlikely.

Yeah, but that's harder when everyone knows Erlang, and one guy goes off and writes something with, say, Django for fun.

But if someone goes "lone wolf" and writes an application in a language he or she chose without informing other members of the team/organisation something else might be going wrong in the chain.

I get the idea you're trying to convey here but in the case of a well-organised team/organisation that won't really fly as everyone will be involved at some point and choices are being made together.

Pair programming comes in handy here.