My problem with Twilicorn isn't focused on whether Hasbro or the show's writers decided to do it.
My problem is that it's a bad idea. It's a bad idea both from a canon perspective, and a more meta fanbase perspective.

Alicorn Princesses in MLP are very, very special characters. They're powerful, occupy positions of authority, and all hints have pointed towards them having immortality (or at least, extremely long lifespans). By making Twilight one of them, they have to address these things, and no matter what there will be unfortunate implications involved.

If Twilight does become as special as her new title would dictate, how do the show writers properly justify her staying in Ponyville? Are they really going to keep her a librarian? The very notion of a super-powerful Princess working in a small town as a librarian is ridiculous, even for MLP.

But, if they don't make her super special, so that her upgrade is largely superficial and doesn't overtly affect her position in society, then what's the goddamn point in the first place?!

Either way, the Twilicorn situation creates a broken base between audience members, where people for and against Alicorn Twilight continue to fight and argue about the integrity of the change until both sides have torn each other apart. Regardless of which side is right, the true problem lies in the fact that the controversy inherent in the issue has irreparably damaged the fandom's relationship with itself.

A Theory has just dawned on me that I believe that Hasbro are doing these sort of decisions so that they can try and get their 'Greatest Creastion' back into the target of their Intended Demographic without losing the more Simple-Core Bronies… which by the looks of things; Isn't, Working.