To be honest I don't see it the same way. For instance if their is only one true path as you say, but multiple ways to get their,then does their special talent even count as special. By your logic that would mean that it doesn't matter what you like, you'll always be special in a way you may not enjoy (see Trouble Shoes). I feel like while that may be what the truth is, it certainly isnt what the show paints the idea of cutie marks to be. Take, if you will, the idea of free choice, if free choice is put into the mix then they may find they love something else, but by your logic they are unable to be truly talented in that field lest they change their set standard of fate. I believe this contradicts the point the show makes more than anything else, as it diminishes the fate of each pony into "you go there and do this and you'll like it". I think it would be much more logical to say that the magic inside oneself happens to grow and change as one grows: a pony could be given something they are attuned with but not enjoy. I think this shows a more likely predisposition to the cutie mark becoming like a part of oneself, for better or worse, and it coincides with how one sees themselves. Almost like a badge of honor for having figured out what they want in life and how it will affect them from then on. Overall, it is meant to be a sign of finding oneself, and as a result can change as does the flow of fate. It may be one river, but the same bits of water might never follow the same branch of said river. It may be one string, but it can be made into many different tapestries and designs. To diminish it by saying that it is predetermined and always set in stone really takes away from the unique nature and unwieldy nature of both living things and the magic we have seen thus far in the show.