The ROCs(Far right nationalists) initial rise is inseparable from the PRC (Communists) as they were working together to Reunify all of China at the beginning.
After the breaking of this alliance by the ROC, purging the PRC to establish their far right nationalist base, they only lasted about 20 years, before the PRC regained far more support, resorting to the ROC ultimately retreating to the island of Tawain while the PRC continued focus on the original goal of uniting the mainland, which it achieved, and has thus ruled far more of China longer than the ROC ruled the sections they held.
By no stretch of the imagination can you claim that the ROC has rightful claim over the whole of China.