By now, if you haven't heard the latest uproarious news, you're living under a rock without internet connection and thus are obviously not reading this blog post, so screw any spoiler warnings.
I'm talking about the whole Twicorn thing, of course. Again, I'm not going to go through all the spoilers and the people's reactions to them, and all the worries they have, running the whole gamut from "understandable and reasonable" to "ridiculously histrionic and pessimistic". I am going to look at two particular worries that I see showing up quite often, though!
First, a lot of these reactions are actually rooted in the fear that alicorn Twilight will blow up people's headcanon about Celestia and Luna - something we also saw, although to a lesser extent, when Cadance appeared in "Canterlot Wedding". It's funny how little the show has actually addressed the origins, nature or exact powers of the Sisters, leaving us to come up with our own explanations and pet theories. On one hand, this is great, as it encourages us to flesh out and build our own cosmologies and takes on the setting and the Princesses, and has led to some of the finest ponyfics in existence. (Some of my personal favorites of this sort are Paradise, Merely a Mare and Eternal; if you haven't read those yet, you really should!) On the other hand, it easily leads to people internalizing fanon - whether their own or not - as "canon", sometimes even impacting your ability to enjoy the show. (*coughChatoyancecough*)
My reply to these fears is to point at "Luna Eclipsed", an episode that blew people's fanon about Luna to Tartarus. Although many got the living anachronism part right, to the best of my knowledge nobody saw the whole Metal Princess of the Night thing coming. And in the end, we loved it! Luna is, if anything, even more popular now than she was before that. So I think the show has earned the benefit of doubt when it comes to this.
Second big worry that I've seen is that Twilight becoming an alicorn is going to screw up the dynamic of friendship among the Mane Six by elevating her above the rest in status and power. And it is an understandable worry; the character interaction between the six friends is the core of the show's appeal, and their Friendship being literally Magic is as important in-setting as it is out of it. People worry that Twicorn will overshadow the rest, or alternatively become a background character to avoid doing that.
To this, I'd say that Twilight Sparkle is already a magical powerhouse, personal pupil and protege of pony Princess, a Heroine of Equestria many times over, and the leader of the Elemental Harmony Squad. The others are no slouches, and are exceptional ponies in many ways, but Twilight quite clearly is the most powerful and important member of the Mane Six. And that does not matter, because the show is about friendship, not power, and even in the adventure episodes the solution is never just having Twilight use her magical power to solve things. If she gets wings and a power boost - and the latter is far from certain - none of this is going to change. Besides, do you really think her personality is going to instantly change if she is transformed into an alicorn? Again, the show is about friendship, not power!
Finally, to take an out-of-setting view of the whole thing, we must always keep in mind that no matter how much we bronies and pegasisters enjoy the show, it is ultimately aimed at the kids. And having the S3 finale kick the Status Quo Is God trope to the curb, and tell the audience that change and growth are inevitable, but that does not mean you have to give up on your friendship; that friendship itself is something that can and should change and grow along with yourself... That would be a very good moral for the kids to learn, and for us adults to be reminded of.
Remember how the show started? Twilight Sparkle, settled and secure in her roles as Princess Celestia's student and as a friendless bookworm in her Canterlot library, is forced out of her comfortable niche. She's sent to a town she's never been to, on a mission she doesn't like or appreciate, to meet ponies she doesn't know nor want to know. And she fights the changes, going along with the letter of Celestia's letter but undermining the spirit of her task because she doesn't see why she should change. Yet it ends up changing her life completely - she gains six great friends, the first ponies of her own age she's ever been close friends with; she becomes the bearer of the Element of Magic, with all the responsibilities and dangers that brings; she even leaves Canterlot, the city where she was born and raised and where she has lived her whole life up until now, moving to Ponyville to live along with her new friends. And it ends up doing her a world of good!
Now, as season three draws to close, she's lived three years in Ponyville, learning about her friends and friendship in abstract. She has become settled and secure in her roles as a member of the Mane Six, as the town librarian, and the voice of reason and authority in crisis situations. Indeed, it almost seems like she could live here for the rest of her life! But life goes on, and people change and grow. It's the perfect moment to shake things up before either Twilight or us in the audience get too used to the idea that this is it, that things should not change.