Abandoned by the US in the 50s, dropping out of UN in the 70s, why hasn't Taiwan collapsed?

Why would they? There economy is strong, you said it yourself. They have a military pointed at them, so does South Korea. That does't mean it needs to collapse now. It is true that in the future they will be absorbed into China as Honk Kong has been or declare independence, but just because it needs to happen does't mean it can't happen peacefully. The fact that Taiwan and China are economic partners now means that both sides have more or an insensitive to handle the issue peacefully.