In a way, I'd argue it was psychologically impossible, less because of what Koreans thought and more because of what Japanese thought. I mean, racism isn't just an affirmation of hierarchy among races, but also of the idea that members of different races are immutably different. As was mentioned above, some of the suggested assimilation measures were adopted IOTL, but at the same time, there was also racial discrimination against Koreans baked into the system. That led to an internal contradiction in Korea policy - on the one hand, they attempted to extinguish Korean as a distinct culture. But on the other hand, they dug their heels in on the question of Koreans as a separate race.
In a way, the premise may well be flawed - in order for Japanese policy to have attempted to erase all meaningful differences between Japanese and Koreans, they'd first have had to believe themselves that it was possible to erase those differences, that all that divided the two groups were cultural trappings. Unfortunately, the prevailing racial attitudes mean that they can't have accepted that as a possibility.