Regardless of how you feel about the GOP, the extent it is allowed to change is the extent to which the ruling class can continue to have their interests served. The DNC is the same in this respect, neither can go against the system dominated by private ownership, but they can slide around as long as they adhere to that.

As for successful revolutions, many. Algeria, Haiti, Cuba, China, Russia, Vietnam, etc. All have delivered much better results for their people post-revolution as compared to pre-revolution. Revolution happens because it's necessary.