Technically, if the Japanese never provoked America or the such, it could've held on Korea indefinitely. Perhaps if America does step in to fight it out in the Pacific without the Pearl Harbor incident, Japan can maintain Korea as part of the peace treaty.
I mean, by 1940 ish, the Korean language was effectively banned, Korean women were forced to become comfort women, Korean men were drafted to fight, Koreans had to switch their name to Japanese, and the younger generation of Koreans were taught that they were inferior to the Japanese and became part of the militaristic culture.
Of course, you can't exterminate the Korean language, as people spoke it in China and there was a huge underground movement that peacefully resisted the Japanese. However, Japan can very well erase Korean history and culture by retaining Korea and oppressing the population.