There's always a ramp-up period for new employees. While it's hard to learn Haskell completely on your own, it's not that hard to learn Haskell if you join a team of experienced Haskell developers. Once they build up a competent team, absorbing new team members really wouldn't be that hard. The only problem here is really that the entire team left -- that would've been a pretty big problem for any complex software system regardless of the language it was written in.