I am a little disappointed where the show is taking Twilight as well... I saw all the symbolism in her name and what she was trusted to do, especially being that Twilight is between the night and day. Plus, assuming Celestia and Luna will not be around forever, who would replace them? It makes sense that they would fashion a pony who is capable of managing magic the way Celestia and Luna seem capable.
However... it always struck me that Twilight would become more like the fabled Starswirl the Bearded, and maybe it would be revealed that she was his descendant. To me, that made more sense. Perhaps it is what will happen and she will be a Princess of Magic. But it seems by all accounts that she's being groomed to follow Celestia to the throne and I am not sure I like that very much. It puts her above the Mane Six in a way that I can't see reconciling.
And poor Luna. I feel very sorry for her. She's always eclipsed in some way (no pun intended.)
No one has ever stipulated that pegasus and earth ponies *can't* achieve the same ascension in status (or even become alicorns themselves). Pinkie Pie was able to quadruplicate and then some using a magic pond, so there are other sources of magic ponies can utilize.
I do agree, however, that some focus should be given to the rest of the Mane Six in pursuing their goals. Rainbow Dash came close at the Wonderbolts Academy, but that didn't end up being resolved. Rarity is still going at it... and Pinkie Pie, Fluttershy and Applejack seem quite content where they are now, but it still leaves many questions concerning their destinies.
One thing I don't particularly care for is how this throws off the balance of the Mane Six. There were two pegasus, two earth ponies and two unicorns. Now there's just one unicorn. That's nitpicky to be sure, but it was a nice dynamic that showed off the races' strengths and differences.