I do not think the Bronies are angry because Twilight is a alicorn, quite the contrary it will be awesome, she will fly totally awesome, but the problem is she has wings and can not use them, stay all day on a throne as Celestia doing nothing .. . I mean what a princess really do in the show? Honestly the princesses are only important for fanfics and fanart, but which their participation in the show?
Celestia raises the Sun, while Luna brings the night. Quite important tasks if you ask me, I wouldn't want to live in a damn cold world. Cadence spreads love (artificial dimplomacy lol) and now Twilight can create magic spells herself. I think it is pretty much like having the ability to research the cure for cancer, for example.
I think these things are quite important stuff right here.
Poor Twilight, all she need is to be with her friends in ponyvile and train her alicorn powers to protect her friends when another crisis come, she does not want to waste her time and power to rule on throne and do paper work, heroes need alicorns too.