> Taiwan is not able to defend itself without US aid.
If you look at a topographic map of Taiwan, it's not so obvious. Rough terrain coupled with the fact that an amphibious assault is needed to even get to Taiwan, and then the troops there need to be resupplied by sea and air (both of which require infrastructure which can be sabotaged), make Taiwan a very good defensive position. Of course it couldn't last forever without external help, but even on it's own it's plenty to cause a massive embarrassing bloodbath.
Island a series of plains with no depth fragmented by rivers from rain + high mountains. Essentially a series of sequential turkey shooting galleries from air. PRC will be the ones blowing up bridges and infra to cut island into piece meal bastions to further restrict operation space of TW. The mountains themselves are incredibly tall, which is a nightmare for defenders limited to light arms against attackers who'll be droning them with relative impunity. The foliage helps, but SAR / sensors tech filling that gap fast. Before/if PLA even bother with landing, they're going to shape conditions to be as uncontested as possible. Which likely means embarassing one sided bloodbath as PLA drone operators in air conditioned mainland offices glassing 100,000s of relatively soft ROCA defenders in plains with no rear except rough mountains rougher than what Vietcon / Taliban operated from. Meanwhile, rest of island - the home front - will have critical infra distrupted, after calories and clean water runs out, they'll be inviting PLA to resupply island vias sea and air. Capabilities of attacker determine whether geography is blessing or curse to defenders. For TW, it's increasingly curse.
Taiwan is a small island, it lacks lot of basic natural resources, like oil, iron ore, etc. One simple thing that China could do is to send their navy to cut off all the supply lines: it won't take long before Taiwan is degraded to third world country. Without the US navy to counter Chinese they do not stand a chance.
You're talking about it being hard to resupply Chinese troops over the air, how about resupplying the whole Taiwanese population?
If you look at a topographic map of Taiwan, it's not so obvious. Rough terrain coupled with the fact that an amphibious assault is needed to even get to Taiwan, and then the troops there need to be resupplied by sea and air (both of which require infrastructure which can be sabotaged), make Taiwan a very good defensive position. Of course it couldn't last forever without external help, but even on it's own it's plenty to cause a massive embarrassing bloodbath.