As others have already pointed out, Sydney has already made an Olympic team, set world jr. records, become one of the top 10 fastest in history in the 400 meter hurdles, set the NCAA record in that event, won an NCAA championship and came .02 seconds away from setting the NCAA indoor 400 meter record and winning that championship (the winner ran in another heat).
With the resume she already has, even if she never competes again, it seems impossible to consider her career "a complete flop." The vast majority of professional track & field athletes will never match that level of success in their careers.
Of course it's possible that she may not reach some of the lofty heights many people expect of her (world records, Olympic gold medals, world championships, etc.) . But not becoming the best in the world does not equal a "complete flop" in my opinion.
As far as Mary Cain, I'm sure she hasn't had the career she expected, but she too has accomplished so much already. Made a World Championship team, won a US indoor title, etc. I hope she regains her form, and if she doesn't I hope she has a great career in whatever else she chooses to do.