It's pretty impossible for China to invade Taiwan at the 50s. First of all, the Communist country was formally established in 1949, and they just fought in the Korean during the 1950 - 1953. The Chinese used the "human flood" tactic, but that tactic is useless in an amphibious assault, since the Chinese have no way of projecting such immense manpower oversea. And the Chinese airforce was small and outdated back then, there'll be little to no air cover for the attack. And what's an amphibious assault without a navy, right? Well, the Chinese navy was close to non-existance back then. No landing crafts what so ever, no Chinese Marines, no armors, no aircraft carriers or support ships, no amphibious assault doctrines.
But for argument sake, let's say the Soviet lend the Chinese their Pacific surface fleet (which ain't much anyway), their airforce, and tanks to conquer Taiwan. And the US just complete ignore the fact that they are still piss off the Chinese intervention in the Korean war, and do a 180 to let Communists do whatever they want in the world. How would the world be without a democratic Taiwan you might say? Probably not much during the 60s and 70s, since China back then was busy hating on the Soviets, and destroying themselves with their "Cultural Revolution". However in the 80s and 90s, the US and China might of been better trading partners without little Taiwan in the way. The Chinese will be able to dominate and bully all her Asian neighbors by 2010 militarily and economically without worrying about a military armed Democratic neighbor to keep them in check. The Chinese will also change their military to be centered around projecting offensive power in the Pacific and Asia, not preparing for an amphibious assault on Taiwan as in OTL.