We just have to replace them again.
Even a large number of people in Taiwan today do not want to be associated with the RoC. There is a growing sentiment to express a distinctly Taiwanese identity.
They were also the "China" entity that was the founding member of the UN. It was only a few years after the UN's founding that the Communist Party won the civil war and formed a new country (the People's Republic of China), yet it took several decades for the UN to officially recognize them as the representative of "China".
The argument to make isn't that they should have a mandate to rule mainland China, nor even that they should be mainland China's representative of the UN. But rather that they should be their own representative at the UN, and have claim to the permanent seat of the Security Council that mainland China currently holds, since it was originally theirs to begin with.
It's a nuclear strategy that'd cause all hell to break loose if that were ever attempted, but is a plausible enough argument that it could likely be forced through if enough parties truly wanted to strip China of their veto power.
