Milotic is just too popular to not get a mega evolution. It is the most requested mega evolution next to Flygon. If it didn't exist, Sugimori would say Mega Milotic was too hard to design. But that is not the case so that means Mega Milotic exists.

The question of the day is... Why save Mega Milotic for a Kalos sequel when it was better to put her in ORAS? There has to be a good reason to save her for Kalos.