Is Duolingo the face of an AI jobs crisis?

I doubt they've completely covered every important language. And from what I've seen, their speech recognition quality is terrible. So obviously there's still room for improvement.

And language changes over time, though maybe not much on these timescales. But even if the curriculum officially covers 100% of a language at one point in time, the language is still going to drift from that snapshot.

Are they really being replaced with AI or is the curriculum just finished?

It was not stellar before (lots of mistake, broken sound samples, nonsensical sentences, all never getting fixed despite reporting a lot), and it got measurably worse recently (all of the above in greater amount), so, no, it was not "finished".