Has Buddhism changed its core beliefs? I don't keep up with Japanese society. Is it in the same social despair we are since they no longer have a true emperor?
When Buddhism entered China, it incorporated a ton of Taoist concepts familar to the Chinese. And when it entered Japan, Buddhism integrated with Shintoism to the point where there were joint Buddhist/Shinto temples. Even today, most Buddhist temples in Japan have a torii gate to show there is a Shinto shrine somewhere on the premises.
Additionally, the foundational mythology of Shintoism came from the first commissioned Imperial histories of Japan (the Kojiki and Nihon Shoki). This was done at the behest of the early Japanese ruler, who wanted some sort of record to prove that he was equal in stature to the Chinese emperors of the time, and the chronicles even state so. (It basically documented the animist myths handed down from prehistoric Japan.)
To me, there isn't anything more human derived than a government commissioned historical chronicle.
Furthermore, Confucianism and Taoism both underwent substantial revisions over time, with neo-Confucianism kicking off in the Tang and Song dynasties of China and becoming *THE* foundation for Korea for at least the last 500 years. Neo-Confucianism created the entire supernatural element (ie - heaven) for this philosophy, turning it into a quasi-religion.
And that Confucian heaven was explicitly modeled after the Chinese government of the time. There was a Celestial Emperor and a Celestial Bureaucracy. Gods were appointed via examination to care for the towns/villages/provinces assigned. And if the gods failed, they were deposed. Sometimes humans were competed in the process, with stories of some humans becoming local gods protecting their territory and people.
Taoism incorporated that wholesale as well.
And back to Shintoism/Buddhism... by the Warring States period, the two were very blurred. Heck, Buddhism had even usurped from Shintoism the 'magic' rituals to create rain at the behest of the Emperor. It was only afterwards, in the Tokugawa shogunate and even moreso in the Meji Restoration, did Shintoism separate itself from Buddhism.