Mao is one of the most successful skaters in the sports and so the title of this thread does sound silly.
Having said that, to address the question that is being asked, I think what made things difficult for her was first and foremost changes to her body, I believe. But she managed to survive them and basically maintain most of her jumps. The only jumps that she really had to give up on due to her body growing up were her quad jumps, I believe. Few female skaters in history have been so lucky as to lose their quad jumps due to puberty.
Mao kept on growing right up to Vancouver Olympic season, and she was 19-years-old then. So this would mean that other girls would potentially need to remain vigilant of growth spurts all the way up to their late teens before they're well and truly beyond the grasp of the puberty monster.
The thing I disagree with is your comment, "it is a fact that her performance did diminish since her prime years." as if it is indisputably true when so many facts and feelings contradict it. She has improved dramatically in the last two years, winning her last 6 Grand Prix events in a row, finishing third and first at Worlds, a new World record in the SP, winning 4CC's by 15 points and becoming the first woman to score 200 points or more in 3 consecutive international competitions. It would be 5 now but for her low score rigging in Sochi. There were setbacks for her at Japanese National Championships and the Olympic SP, but the last three performances are the best she has ever had.
Most people agree that Mao was underscored at Sochi, yet even held down her tech score was still as high as her best tech score in the Grand Prix Final 2007, for what is surely her best tech performance BEFORE Sochi. As for artistry, Mao has perfected a wave-like flow in her arm and hand motions that is constant but especially noticeable when exiting her jumps. This surpasses the artistry she achieved in that Grand Prix Final 2007. Furthermore, her step sequences have reached a level of transcendence that is simply divine. Again, her two performances at Worlds and her incredible number of tweets praising her Sochi performance including the unprecedented number that claimed they cried or had goosebumps) only adds more credence to the fact that she has enhanced the quality of her performances, so that 2013-2014 is her prime year. And while she may have lost to Kim overall at the Olympics, there is agreement in the court of public opinion (social media, tiwtter, you tube and forums) and even among many Olympic skaters, like Tara Lipinski and Elvis Stojko, that Mao had a superior free-skate at Sochi to Kim. I think Mao's coach Sato sums up best what distinguishes her performances now from pre-2010.
"4 years ago she came to me as a two-time World Champion and to be honest there were many days when I was unsure what I should do, thinking this way and that way. Finally we reached a point where we are on common ground, which is right now. Mysteriously she has become someone who can produce something profound."
This depth for me is her consummate artistic and even common human expression she brings to her skating that is quite exceptional and rare, and in my opinion, likely built up from her years of suffering to improve her jumps. her Olympic heartaches, and most importantly, the tragic death of her mother.
Finally, we have the toughest critic of all, Mao herself, who admits “I achieved my targets and feel a sense of completion." Even Mao is claiming success. The only thing that could diminish is the world of figure skating with Mao's absence.
Mao was undefeated on the GP the last two seasons in a row. She had some of her best performances from a technical standpoint this past season and was consistently hitting at least 5 triples whereas the season before her wins could be attributed to superior PCS.
She had a rough FS at nationals but everyone has an off night...but on the whole, Mao was "on" this past season which is why her meltdown in the SP in Sochi was such a shock. But she rebounded with a brilliant FS there and went on to throw down two excellent performances at worlds.
Mao struggled post-Vancouver with reworking/relearning jumps as well as outside factors (the earthquake in Japan, losing her mother). She basically bottomed out; yet she climbed her way back to the top. The fact that she continues to compete and win is a testament to what a resilient and amazing competitor she is. She's the only one of the Big Three who has competed year in and year out for the past two quads.
Was the level of competition in the GP for the last two seasons that strong?
That aside, I'll correct my original statement. I do remember a lot of teeth gnashing from posters over her Olympic prospects prior to the Olympics though.