I always found it strange that people believe in the existence of a situation where PLA tries to land on Taiwan while US is actively engaged with PLA at the same time.
If I had to guess, it's probably because these predictions are coming from Western institutions who have spent the last 70-80 years thinking about a Korean or Warsaw Pact drive across the DMZ or inner-German border, respectively.
An actual invasion of Taiwan was such a low possibility for so long that no one ever gave thought to how it would play out. So for these people, they just replace Korean infantry or Soviet tanks with Chinese amphibious vessels and expect them to zerg rush across the Strait, which is the new proverbial DMZ/Fulda Gap.
You see this with AirSea Battle. My understanding of it is that it is quite literally AirLand Battle but on the sea, using shore-based ASMs, subs, destroyers etc. networked to a network of eyes (sats, recon planes, drones, etc.) that are expected to hit Chinese ships as they "advance" into the Pacific. No consideration is given to what would happen if the PLAN just hung out around Taiwan, how the US would break through a Chinese blockade of Taiwan, etc. The strategy hinges on a Chinese seizure of the
entire 1IC.
Studying WWII is important here. The US did not bum rush the Philippines, it took its time and cut off the islands first, mostly with submarines. Every major amphibious assault was preceded by a major naval and air battle. Only in instances where a target was virtually undefended (US, British, and Dutch forces in most of the Pacific were abysmal in '41) did amphibious assaults tend to come immediately, but even then there were mop up operations and air strikes well before any troops landed.
I'd argue the JSDF, for as little contingency planning as it is allowed to do, is a little more clear eyed in this regard. The reason they say a Taiwan contingency would be an threat to Japan warranting intervention is that on account of being founded by former Imperial military officials, they have the institutional memory of the US occupying outlying islands around the main target and setting up radar stations and emergency landing fields around them before going in. Yonaguni and Ishigaki make perfect targets in this regard.
That said, it's still a silly notion, part of why China is building aircraft carriers is probably to avoid having to do that and being able to fully cover the Pacific side of Taiwan (land-based fighters could do it too, but there might be gaps depending on mission cadence).
If US fires on PLA, amphibious landing destination will be Okinawa and other 1IC islands, not Taiwan.
If US doesn't fire on PLA, armed landing won't be necessary with a blockade.
Ergo, armed amphibious landing exercises are for Okinawa, not Taiwan.
This is the strategy they used in the CMANO DLC Chains of War lol.
On a serious note, boots on the ground in Japan may be short-sighted. Any war should be fought so as to end as quickly as possible. If Taiwan is blockaded and bombarded, regardless of the will of its leaders, its military will be destroyed in a matter of time. Then the PLA can land with relatively little resistance. If that happens, Japan and the US will have no choice but to give up.
But how is China supposed to end the war if there are still troops engaging in combat on Okinawa? Or Japanese and American aircraft actively harassing naval and air forces, perhaps even launching sporadic cruise missile strikes on the mainland?
The idea of seizing just Okinawa reminds me of the US decision to "just" deploy troops around Danang Air Base. It's an escalation with no off-ramp. If the ROC falls, will the PLA just suddenly evacuate Okinawa? Won't that look like a defeat in the eyes of the West and Chinese public?
Seizing Okinawa will also trigger the US-Japan Mutual Security Treaty. The US will be obliged to fight hard until Okinawa is retaken. Taiwan is a political commitment, but defending Okinawa is a legal commitment of the US. Losing it would raise serious questions about NATO and other alliances. It's also covered by US nuclear deterrence, unlike Taiwan, and therefore its permanent loss could result in a nuclear exchange.
On this forum, I've seen a hypothetical war against Japan be compared to the 1979 war against Vietnam; a brief action that bowed Vietnam from challenging China. The two situations are not comparable, IMO. Soviet support to Vietnam was limited, it was essentially a backwater in the eyes of the CPSU. The USSR was not treaty bound to defend Vietnamese territory. The US is the opposite of these two things to Japan; the two are strong allies and the US is obliged to defend its territory. Besides defending South Korea, defending Japan is literally INDOPACOM's biggest responsibility. The Soviets never had anything like that with Vietnam.
If there are going to be boots on the ground in Japan, it will require a significant expansion of the war. It will require defeating Japan in much the same way as Taiwan. That would be all fine and dandy if there was no security treaty with the US, but there is.
IMO, long-range strikes against Japan are enough. The JMSDF cannot replace itself, it's lagging in recruitment goals as it is, it would be impossible for the Escort Flotillas based at Sasebo and Kure to be reconstituted if they were fully destroyed. The JASDF has the same problem, although attriting the JASDF would be harder given the distances involved. Japanese politicians, for all their nationalist bluster, are highly opportunistic. Look at Abe's lukewarm response to Obama's efforts to get American allies to rally round the flag against the seizure of Crimea. If what remains of the DPP has been removed to a mainland MSS blacksite, the Kee Lung class are at the bottom of the sea, and CCTV is airing images of smiling PLA troops handing exhausted Taiwanese citizens packages of food and water: and then this is contrasted with grieving family members of the crews of sunken JMSDF ships, smoke rising in the air from repeated strikes on Japanese air bases, etc... a Japanese politician will be able to see the writing on the wall. Those guys may dress up like Tojo but they do not have his will.