I'd say stability is a big piece. If the ROC moved into some of these territories there would be war and even if they won there would be people left over within those territories who would be willing to fight to change it back. If the change of territory doesn't lead to more war, or even reduces the chance of future war then I'd be more ok with it.

The people who live there, their conditions and what the competing countries have been proven to provide. Like, there could be a situation where it might piss off nationalists but a change in ownership could improve quality of life for the people there. Obviously the popular opinion bit is important to. Consent to governance is important.