How does that make it a "necessity"? It's not for China to decide? This is the reasoning Russia uses when invading neighboring countries. To "protect" russian people and claim that <insert part of country> are russians anyway and want to get annexed (still wouldn't make it right). If someone wants to join Russia, they should move to Russia.
(Or maybe it could happen through some longer and slower political process. And the country as a whole should agree, with a lot more than 50% agreeing, to a unification.)
> The Chinese way of thinking is that only after a group has been fully Sinicized (language, culture, identity) can they be considered “one of us.”
Like above, I hope you're not implying that a culturally similar people in another country #2 somehow gives country #1 power over it's sovereignity.