Was there a legitimate fear that not doing so would necessitate an invasion of the Japanese home islands? Couldn't the US have continued either conventional or nuclear bombing raids indefinitely? Butting up against the 20 year rule, I wonder this because of the relative historical revisionism in Japan versus Germany, where it would seem there is a more contrite societal historical perspective. As a secondary question, did leaving Hirohito in place lay the ground work for this revisionism?
This thread covers the basics of why he wasn't tried as a war criminal. My write-up in it discusses it a little bit. The basic ideas are this:
The issue was not that they were afraid of prolonging the fight (well, not only that, it did factor in obviously).
The US were afraid of losing influence in Japan.
Protecting the Emperor meant they could use him as a figurehead to keep US interests in Japan protected
The Japanese were actively working to shield him, as well, to ensure he wasn't tried. So why keep power, besides the above reasons?
Well, he didn't, to be honest.
The Potsdam Conference didn't mention the Emperor being removed for the aforementioned reasons. However, using him as a figurehead continued to be helpful, as the Americans were greatly involved in drafting the new Japanese Constitution. Under that new constitution, adopted in 1947, the emperor lost most of his power, and mostly retained ceremonial duties. In that way, he was emperor...but not really emperor. It was useful, it kept the Japanese public somewhat more complacent, it helped the US gain influence, and they didn't have to fight as much to get the new constitution implemented that would help remove the military "clique" that they so disliked (and which they said was a huge factor in why the war even began).