I agree with your idea of when the show will end, @ . people are going to hate me, but i feel that if your theory is correct, i would add on with the other 4
Fluttershy: um...i don't really know. maybe she becomes in charge of the animal helpers for winter wrap-up or is the gardener for the castle?
Pinkie Pie: manages to become friends with all of equestria and hosts the grand galloping gala
Twilight Sparkle: well, she's an alicorn. perhaps she gets a kingdom? although luna doesnt have one... she saves everypony from an ursa major attack?
Apple Jack: gains ownership of sweet apple acres...idk, cant think of much for her.
these are hypotheses and probably wont come true anytime soon, but still it's fun to guess. also, I'm hoping for at least 10 seasons. 8 and i will be content, if they are good enough.