I'll address both posts in a single reply
Really? Even if those are defended with SAM and point defense weapons? US land attack weapons are subsonic, which make it easier to engage. The fact that those islands are relatively small make them easier to defend against land attack weapons, paradoxically.
I think that given their size, and the fact that they are so close together as to be able to offer effective mutual support, as well as the PLA strengths that could assist them from the mainland and Hainan bases, once fully operational, those bases will not be that vulnerable, and most certainly will not be easy to neutralize, even by the USN.
I expect these islands to eventually have as extensive underground facilities and bunkers as they will have surface installations, if not more so.
The PLA pretty much design these islands from the ground up to be hard nuts to crack, and short of launching amphibious assaults to storm them, I don't think anyone will be able to neutralize them from long range bombardment.
The airstrip will be the most vulnerable, and will probably not be able to remain operational consistently for long under heave saturation attack (which, incidentally, might be one of the reasons the PLA has developed a keen interest in VTOL fighter aircraft in recent years).
However, if they build as comprehensive a set of underground facilities as I expect, the PLA garrison would be able to hide and protect enough long range SAMs and AShMs to make it extremely hazardous for any hostile naval and air forces to operate close to them while they remain in Chinese hands.
In effect, these islands would form a bastion in the SCS as China's first line of defence.
No hostile naval or air power can operate with any degree of safety within the SCS while the remain operational, in effect shielding bases and facilities on the mainland from attack.
Taiwan would serve a similar purpose for all of Southern China, which is one of the main reasons the US military is not at all keen to see it ever return to Chinese hands, but that's a different discussion.
But anyways, these island bastions would in effect help to create a "safe zone" (as safe as could be managed in a war zone between near peer superpowers) behind them, giving the PLAAF and PLAN an operating theatre largely safe from heavy enemy attack. The PLAN and PLAAF forces able to operate inside that "safe zone" would offer the islands massive logistical and direct arms support to make them even more formidable.
They will, of course not be able to categorically stop hostile assets from breaking through into the SCS waters behind them, but any naval or air forces seeking to do that while the islands are still in Chinese control and operational would face huge risks and almost certainly come under heavy, sustained attack on the way through.
If they can break through, they will be largely cut off from other friendly support, and run a high risk of being forced into a position where they are caught between the anvil of those island bases and the hammer of the full might of the PLAN and PLAAF.
China's coastal areas contain the lion share of its industrial and financial assets as well as the bulk of its population. Its leaders have long fretted about the ability of foreign powers to put those core regions of Chinese wealth and power under direct threat of military attack.
These island bases are part of China's long-term strategy to deny any hostile power the ability to launch first strikes against those areas.
If during peacetime, the islands consistently host a significant contingent of fighter aircraft, and associated aircraft (AEW&C, MPAs etc) as well as feature their own organic defensive infrastructure (a SAM unit or battalion even), and if they regularly train with the navy and air force so they all provide support for each other where the mobile naval ships and mainland based air force units are able to operate in an integrated manner with the islands, then what you suggest could definitely be viable.
However, in the foreseeable future I can't see China would be willing to militarize those islands to that extent, which in my mind would make them very vulnerable (by design) in a high intensity conflict.
If China eventually did decide to militarize the islands significantly, then they could stand a fighting chance in a high intensity conflict, if supported by mobile naval forces and mainland air force units.
The fundamental limitations I see with the islands as fortifications, are that they are fixed, they are relatively small, and they're far away.
-They are fixed, meaning the opfor already knows where they are, which drastically shortens their necessary kill chain. Being fixed in itself isn't necessarily a significant limitation, but if paired with other detrimental factors (below) it can increase the difficulty of defending the islands.
-Their small size means they have less space for supplies, armaments, spare parts, equipment, and soldiers to man defensive units, and little to no space to hide defensive units such as TELs. The small size of the islands not only makes them reliant on resupply to perform their roles, but it also concentrates their vital structures close to each other, allowing the enemy to "concentrate" their firepower in a way where they know they will be likely be hitting something within the box. (Also, I'm not sure if those islands are able to host bunkers. The islands are reclaimed, so I'm not sure if their soil composition(?) and what not allow bunkers to be viable. I suppose they could build hardened shelters, but regardless of whether bunkers are present or not, I do not believe that can make up for the limitation of their relatively small size of the overall islands)
-They are very far away, and isolated from the Chinese mainland which would be the main source of resupply and support forces during conflict. If a reliable supply chain cannot be established, then the efficacy of the islands drastically falls, even if they may be able to support each other independently in the region for a while.
I think the above limitations can only be mitigated through a more consistent highly militarized presence on the islands during peacetime, as well as greater Chinese air and naval capability to allow the islands to be supplied and to provide mutual support for each other. I cannot see China stationing a major military presence on the islands in the near term, which is the basis of my position.
If, in the preceding stages of conflict, there is only a minor military presence on each of the islands (say a couple of MPAs, and a point defense unit), then that makes them very vulnerable.
If, in the preceding stages of a conflict, China manages to ship major defensive units onto the islands and also to send a large contingent of aircraft to the islands before any shooting starts, while also being able to send in a major naval task group near the islands with a persistent, enduring air force presence over the SCS in support, all working with each other, then there is more of a fighting chance.
So, I'd say that the big, first hurdle to viably defend the islands lies in whether China is able to station a major military force on the islands prior to the outbreak of hostilities, and the second hurdle is whether China's air and naval power will be sufficient to reliably resupply the islands and whether China's air and naval power has the reach and the endurance to remain around the SCS in a way where their presence helps to support the defense of the islands in the first place.
