Even in the early 60s about the only country willing to defend the ROC was the U.S. Only 38 Countries didn't vote to extend the PRC the right to take China's Seat in the General Assembly (and Security Council) in 1971, of those the only ones that had any sort of status regarding a potential defense of Taiwan were Australia, Japan, the Philippines and New Zealand*, with only Australia being even theoretically able to assist (New Zealand lacked the available forces, outside of potentially a symbolic company or battalion, the Philippines were a critically important base, as was Japan, but neither would have been a source of military power). Even the other closest U.S. Allies, Canada and the UK, weren't on board.
The U.S. single-minded willingness to defend the ROC is absolutely unique and had been for over half a century. Even the U.S.' actually willingness to go to the mattresses is anything but certain, as has been the case since the late 70s.
*Neither the DPRK or ROK were UN members in 1971.