There's been no backlash from the top dogs about AG being retconned, which is as good as them approving the retcons, meaning that they could be in on it too, or worse they don't actually give a damn because they which in turn, means that the asylum is being run by inmates who want to shove their biases down the viewer's throats! To the showrunners, Ash Greninja is a betrayal of the true history of the Anime, thus it needed to be erased, just like Amour!
Now that I think about it, it's starting to feel like that Serena was in JN105 only to shut up people who asked for more Serena appearances. Noticed how all the people who hated her, loved her character in this episode? Even while supposedly shilling Kalos, they did it in a way that panders to people who depise XY! In fact, the treatment of anything regarding XY in JN indicates that the production team regretted actually doing XY to begin with!
As for Dawn, they're definitely bringing her back permanently, because Piplup merch sales have skyrocketed to the point where Piplup is now being considered a full-blown shillmon and she's a popular female character! In hindsight, Pearl probably won't be canonised because the writers are too chickens**t to actually commit to anything as serious as romantic relationships but Dawn will still be permanently attached Ash because: "ThEy'Re IcOnIc!!!" and the writers love the two being together!