I don't know for sure, but didn't Yuzu get the flu sometime during/after 2013 4CC? The Olympics he actually did well for all his SP, but I think the pressure and the late schedule for the men after the SP made everyone tired. 2015 WC... well that was more health related and lack of training time.
That's not always true... he came in 3rd at his first worlds without going to 4cc (last competition was nationals)... the year he went to 4cc before worlds (2013), he underperformed (at least for his standards) at worlds. I honestly believe it would be a lot better for Yuzuru to rest after nationals and train in preparation for worlds instead of going to 4cc.
Yuzuru did under-perform at 2012 WC. His SP was not good. Fortunately he fought back in the LP. But his overall performance at 2012 WC was worse than his 2011 GPF and National performances.
He was sick at 2013 4CC, but he was also sick at 2012 GPF, yet his performance at 2012 GPF was much better than his 2013 4CC performance.
He had a good SP at the Olympics, but his LP at the Olympics was not good. He did get some competition experience at the team event (short program only) before the individual competition at the Olympics. I do think his team event experience helped him to perform his SP well at the individual competition.
At 2015 WC, he didn't have enough training. But he could land all the jumps in practice just before the competitions and he watered down the content. The mistakes on the 4t (step out and fall) were due to his ankle not fully healed. But the pop of 4S was a mental mistake because he hesitated before he jumped. The pop of 4S was what costed him the gold, not the fall on the 4t (because Javi also fell). So I do think it was his mind that was not ready, more than his body.
Every athlete has health/injury related problem. Yuzu's health problems are not more severe than other athletes'. (He is physically not a weak person.) He is a top athlete. He can't and he doesn't use health reasons to justify his sub-par performances.
Also, I don't believe anyone is born with nerves of steel. Athletes are humans and humans have emotions and weaknesses. Humans need to personally go through the hardships, setbacks, difficulties and trauma to mentally grow stronger and more mature (to handle the increasing pressure/expectation). Nobody can become mentally strong by living a comfortable life with no hardships. His last season may be seen by many people as bad or unfortunate, but I actually see it as a great opportunity for him to mature and become mentally stronger. I'm glad he didn't let that opportunity go wasted.
There are 1.5 months between National and 4CC and another 1.5 months between 4CC and world. That's plenty of time of rest and training. He only had 3-4 weeks of training between Skate Canada and NHK. I think 1.5 months is long enough to prepare for a high level competition.
The thing is there are costs and benefits associated every decision. Every individual makes his/her decision weighing the costs and benefits associated with the decision. The problem is many people see 4CC as a lesser competition. People don't see much value in competing at 4CC. So the extra travel and fatigue becomes a big cost. If the decision maker sees no value in 4CC, then the cost will outweigh the benefit.
I'm sure if European Championship is held in Asia, nobody will skip Euro no matter how far away the host country is and how tired he/she is due to the travel because everybody sees Euro as a prestigious competition. (But 4CC in the recent years has a deeper men's field than Euro even if the competition is perceived as less prestigious. The men's field in Euro has been very weak for many years. Winning in a deep field is more meaningful and shows the skaters' skills much more than winning in a weak field.) If the Olympics is held in Asia and WC is held in North America 1.5 month after the Olympics, I'm sure nobody will skip the Olympics in order to prepare for the WC. They will skip the WC after the Olympics because the Olympics is more prestigious.
Many people see the time and location of 4CC as a disadvantage. But IMO, the time and location of 4CC is the best value of 4CC. Yes, 4CC is far away from the USA and Canada and it is held between National and WC. NA Skaters will be more tired due to the long travel and jet lag, but they have to go through the same tiredness and jet lag when they go to Korea for the next Olympics. 4CC maybe the best practice competition for the next Olympics, not to mention the 2017 4CC will be held in Korea.