I had typed a looong post on this, but accidentally lost it before I submitted it, which turned out to be a good thing, because I posed a question which I have now realized the answer to. The question was: "Why is the 'China in America' thing such a big deal?" My basic point in the lost post was that the Chinese getting to the New World, while not the most likely thing, was still a reasonable alternate history possibility, and was actually probably more likely than some other things we commonly see in the game.
And then I realized three things. First, the problem is not that the Chinese get to America sometimes (though it may be too common). Rather, the problem is that if they do get there, they beat the Europeans to the Atlantic coast. And there were two reasons why people make such a big deal of this occurance. Compare and contrast this with some of the other non-historical occurances, like the HRE minors getting gobbled up or the common situation where each Irish province is owned by a different country (both of them less likely, IMO, than Chinese expansionism). You'll see 2 key differnces:
1) Player actions: The human player can gobble up HRE minors just like the AI. But the human player, playing as China, really can't beat the Europeans to the east coast of the Americas.
2) Possible fixes: It's not real obvious how to fix the too common annexation of minors and still allow historical annexations without killing gameplay. Most of the fixes I've seen proposed would probably be worse than the problem they seek to cure. But the Chinese expansionism has an obvious fix that any player can see--make the AI subject to the same naval attrition as the human player, and make it smart enough to deal with it.
So the whole thing really comes down to how hard is it to make the AI able to handle naval attrition. I have no idea how hard it is, other than I can't do it. If someone who actually knows how to program something like EU has any ideas, I'm sure Paradox would be interested. I'm sure it's not helpful for someone who doesn't know how to do it say "Well, how hard could it be?" If you know how to do it, you can ask that question, but someone like me who doesn't know how to do it, well, I don't have the right to ask. I do have the right to say that I hope they can find a way to fix it, but if they don't, they have still made a great game for us to enjoy, and I appreciate that.