In charmed, The angels of destiny are Godlike beings who controll how the natural order works, however some destinies are not fullfilled in the series for various reasons.
forgetting the angels however, The destiny of the charmed ones was that they would be the most powerfull withces the world had known, but who were the charmed ones destinied to be? Prue, Piper, and Pheobe or Piper Pheobe, and Paige? according to the comics, Paige was not meant to even be a charmed one, and possibly wasnt even supposed to exist, though it seems odd that while destiny was altered in prue's death, Paige was there coincedentally with telekinetic powers like her sister due to her mothers affair with her whitelighter
the explanation given by prue of course is that she purposely led them to meeting eachother, and passed on the power of three to paige herself, but If prue was destined to be the only telekenetic charmed one, then why did she die (other then of course the actress bailing on the show after 3 years)
in the episode where they go to the future, prue is still alive, and her powers are super advanced as with her sisters, the explanation for this level of growth not occuring in the true timeline is that prue was replaced, but what what caused prue to be replaced, this sight of the future affecting their decisions in the present, causing the issue with shax killing her, thorugh some dominoe effect, or possibly the fact that pheobe can see and alter the future mixed up their destiny. time travel and pheobes power of premonition are the 2 reasons that nothing in the charmed universes "destinies" can possibly be 100% accurate, for instance, Pipers daughter was born too late and was therefore probably not the same girl seen in the future, however if that future was the destined one (which included prue) then wyatt would not have been born at least not at the same time, and possibly not as the same being, but him being twice blessed, and being the possesor of excalibar would be two destinies that would be put under question, would those destinies have been put upon alternate future melinda, or would they have been postponed till his actual birth considering he was later born as the same being but younger.
any prophecy can be avoided aparently due to the fact that the prophecy its self can serve as a pre-warning system, or can indirectly cause its self not to be fullfilled,
also it would seem that death would be considered a destiny, yet the charmed ones seem to constantly find ways to cheat death, yet they did not look for a way to do so with prue, who they inevitably let stay dead, yet she later cheated death herself by possesing another witch.if destinies were set in stone, then deaths would be too,
sorry this is kind of a long, really poorly written blog, but I was just trying to put in some evidence and key points