Let's first not talk about a superpower like the United States, but just consider it from the angle of a typical country--Thailand, for instance. Thailand and the mainland have diplomatic relations, and they were established with an understanding of the mainland's policy about Taiwan. If Thailand were to recognize the Republic of Taiwan, then it would violate this policy, and the mainland would definitely sever relations with Thailand.
我們先不管像美國這樣的超級大國好了，就從一般國家的現況考量，舉泰國為例，泰國和大陸也是在這樣的立場上建交的，如果泰國承認台灣共和國，那就違背泰國和中國大陸建交的立場，大陸就一定會跟泰國斷交。