my theory is that, since the elements were redistribuitrd in the mane 6 and Twilight became princess of frienship, all the concepts Alicors are related to (big concepts like Friendship, redemption and such) are being redistribuited too. As an instance, I headcanon Flurry Heart as princess of Joy (though, I'd love to see her turn evil- maybe corrupted by Sombra even) and I guess that Starlight would rappresent Redemption or Forgiveness.

I don't really think alicorns can retire from their positions... because I identify these positions as a part of them - that's why they're immortal, in my opinion. The only way an alicorn could "Lose it's job" is probably if it got killed or incapacitated in some way. There aer so many things about alicorns that are still a mystery and I hope season 6 will explay them