Things would be even worse if USA leaves Japan to be the BOSS in East Asia. World War 2 stills haunts Asia.
Bad blood: China-Japan timeline
Bad blood: China-Japan timeline
Also I doubt USA will be happy to let Japan have be independent politically and militarily from USA. USA still remembers Pearl Harbour and USA doesn't want Japan to be a threat to the USA in the future.
Anyways :
The Japan That Can Say No
The Japan That Can Say No - Wikipedia, the free encyclopedia
What is wrong with Japan being the Boss of Asia? Yes, WW II still haunts Asia, but let me reminds you western nations had also ravaged Asia for more than a century. Even though I don't agree with the action of the Japanese had taken during the WW II, but one of their objective was to free Asia from western colonist powers and united everyone under one Asia. And as today Japanese can certainly be the role model of Asian countries, and the disciplines of its citizen had displayed will take more than hundred years for any other nations to learn. If you have been to any game or event that have Japanese hold you will understand what I mean. Just take a look at how people behave in Shanghai expo this year and how people behave in Aichi expo 2005. This has nothing to do with the income level or anything else in that matter. China passed Japan for GDP this year, and maybe sometimes in this century will come close for GDP per capital with Japan, but it will take much longer for people in China to reach the maturity that Japanese have shown to the world.
Since we are on this object, let me tell you this. Even the language itself, China in the history had brought Chinese characters into Japan through Korea, but for modern Chinese language its was heavily influenced by Japanese. Words like service(服务), organization(组织), discipline(纪律), politics(政治), revolution(革命), government(政府), party(政党), guideline(方针), policy(政策), application(申请), solution(解决), theory(理论), philosophy(哲学), principle(原则), economics(经济), science(科学), business(商业), officers(干部), health(健康), socialism(社会主义), capitalism(资本主义), communism(共产主义), law(法律), feudalism(封建), republic(共和), aesthetics(美学), literature(文学), abstract(抽象) and much more were all imported words from Japan. There is a really good book about the relationship between Japan and China on this in Chinese called "100 Excuses". There is a more complete list of these words. So yes, I think they have the right to be the leader of Asia for now until let's say China can compete with Japan in all aspect including individual levels.
Japan is the only one country in this world had suffered from nuclear attack. Militarism is not going to be very popular among its people, even though there some people from time to time will make some noise about it, but they are the minorities as every country has those. The sooner China and Korea can move beyond what happened in WW II, the sooner Japan will move towards a better directions. The unfair mistreatment and distrust towards Germany after WW I did not helped Germany becoming a more peaceful and friendly country, it actually pushed it toward the other direction. It is a lesson people in Asia should learn.