As long as the US is a naval/aireal world power, and as long as the US is involved in the region. China cannot take Taiwan.

They don't have the navy or airforce numbers or quality to perform a sustainable landing operation while facing a naval/aireal threat from other countries.

A regional power like Japan's navy alone, if they got involved, might be enough to counter China's invasion simply by threatening troop convoys and supply convoys throughout the invasion attempt... and obviously if the US is involved basically every Chinese ship above water would be sunk in a matter of a week or two.

It also realistically isn't worth it, even in a world China had the navy and airforce to conquer Taiwan they'd lose hundreds of thousands if not a full million men in the attempt. All for a Tiny island they'd have to level with bombs and make more or less worthless just to obtain. Losing that much of your army/population all for an island just isn't logical.