The Elements of Harmony haven't been needed in such a long time and still sit in their place at the Tree of Harmony. It's been many years since Equestria has been at peace. However, Queen Celestia and Luna have taken notice of things yet to come and have debated every option they could take. Tirek was brought up and Celestia decided that trying to reform him would be the best option they would have. The risk was heavy, but it was a risk they needed to take.
Based on my own Next Gen characters
Keep in mind this is how Celestia and Luna look like in this AU
Next Gen Queens