It's all about timing though. They wanted them "out NOW" in the 50s IIRC. If everything was Dominionized in the 20s or 30s then by the time we get to the 50s they really could just leave peacefully IMO.That gets you more war and misery becaue a lot of people want the white colonialists out NOW! Or better yet yesterday but now is the next best thing and they'd be perfectly willing to take up arms to make it happen.
Training the locals also leaves you with the issue that they might be seen as collaborators of the colonial powers, that they dont intend to leave, just give you a local figurehead and continue business as usual.
The only real alternative to leaving instantly is giving them full rights and not leaving at all.
Of course this does require both sides to have good behavior and intentions.