Still not interested. They're not why I watch this show. The creators can give A episode to the background ponies, but the 100th episode milestone had BETTER be dedicated to the six mares who made this series what it is.
Imagine if the 50 Year Anniversary of Doctor Who wasn't about The Doctor. What would be the point? The same goes for any other series milestone of any show. Milestones should be focused on the main characters.
It's the same for the 100th episode of FIM. What is even the point if it ignores Twilight, Applejack, Pinkie, Rarity, Rainbow, and Fluttershy? They're the main characters. I repeat, milestones should be about them. The 100th episode can be so good that it causes world peace, and I still won't watch it if they're not the focus.