I saw that episode on TV, and I never was more disappointed, because I didn't want it to happen, even more considering that Twilight would not have a real occupation, because Celestia and Luna already did the "goverment work". However, in the last season they introduced the "retirement" concept (Luna spent 1000 years in the moon and now wants to retire), to justify her rise as the ruler of Equestria. Such a plot twist, I actually wanted Twilight as a counselor or something related, not literally the new queen.