XT and I have had this debate before and we disagree. I think that without American assistance, it would be possible for China to conquer Taiwan if they were willing to pay the price for it.
People talking about amphibious invasions tend to focus too much on how Americans do it. American military doctrine is to plan things out, load up on equipment, and minimize friendly casualties. That may be the best system if you’ve got the resources but it’s not the only way to do an amphibious invasion.
If you’ve got a lot of troops and a halfway decent amount of equipment and you don’t care how many of them and how much of it you might lose, you can throw a lot of troops across the water.
People forget that the Nationalists originally controlled two big islands, Taiwan and Hainan, after the communist victory on the mainland in 1949. But the communists invaded Hainan in 1950. They basically loaded up over a hundred thousand soldiers on civilian boats and sailed them across the strait.
Now I’ll grant this wasn’t the same situation. But I don’t think the differences are insurmountable. Crossing a hundred miles is more difficult than crossing ten miles - but they’re comparable (the hardest part is that first mile). The Taiwanese navy and air force are better equipped than they were in 1950 - but so are the Chinese navy and air force.