[ShadoWarrior]

The PLA was crap back in the 50s, too, and they still kicked our asses in Korea. Numbers do matter. Even if each individual plane, tank, and ship is a decade or more less advanced than its opponents, enough of them will cause a world of hurt. Sure, we'll get a 6 to 1 or 10 to 1 kill ratio in the air, somewhat less than that at sea or on land. But when the enemy has a 10 or 20 to 1 advantage in numbers...

And it doesn't matter where they get their stuff from. So what if they don't design the stuff themselves, but get it from the Russians (and steal it from everyone else)? The Argentinians were using bought planes and missiles when they sank British ships in the Falklands. America's superior technology, training, and doctrine can only go so far to counterbalance enemy numbers. And that's under ideal conditions, which also assumes that no one on the US side makes a major blunder.

China isn't Iraq. Their units won't just roll over and surrender when they run into the big tough Americans with their fancy toys. And the Chinese integrated air defense system is second only to that of Russia's. The US got a very painful taste of it back during Vietnam, and they've kept up with tech improvements ever since. The Chinese aren't stupid. They know what the US has, and will be prepared to deal with it.

US units would be at the end of a very long logistical tether. China would be fighting in their front yard. It's so close that logistics isn't really an issue for the invaders.

As Webstral said, the main calculation is whether the Chinese are willing to take the economic hit. That they haven't been willing to, or risk a confrontation with the nuclear-armed US, is all that keeps Taiwan free. Even so, Taiwan has been fairly well Finlandized.