There is no such thing as constant line of defence in naval warfare.
Depends entirely on the situation (the term 'ships of the line' is old, but the philosophy behind it may still have value today), but it was just a turn of speach in any case, so seems like a rather pointless nitpick.
It's question of:
(1)speed and it's sustainability
(2)maneuverability(yes, it matters)
(3)acoustic signature
(4)availible countermeasures and their effectiveness
(5)survivability to underwater hits
and tons of other 6,7,8,9,10 and ce te ra.
You can perform ASW helicopter carrier work even from PANAMAX. Problem is, you'll just get your torpedo and what's it.
LHD is obvisly better than container ship, but in no way it's proper blue water combatant. Yes, even America-alike.
Sigh, how many times must I say this? But the whole point is that the LHD is not supposed to be running down subs itself! That's what the 056s are for!
The LHD is there as a flagship to provide co-ordination as well as the rotor wing ASW assets the 056s themselves lack the hanger to house for long range operations.
It does not need to be stealthy, or armed specifically for ASW or anything else on your list other than speed, but that would likely have been a requirement anyways to keep up with carriers.
To add such capacity to a ship like an LHD is either a complete waste, or shows a total lack of confidence in your ASW assets that such high value assets might seriously be expected to have to deal with enemy subs directly.
p.s. if you dislike japanese ships(which are, in any case, closest currently availible) - you may just as well look at italian carriers.
I don't dislike Japanese ships because they are Japanese, I just think using their helicopter carriers as some gold standard is totally flawed because their 'helicopter carrying destroyers' themselves are a fundamentally politically compromised design.
The Japanese didn't want ASW carriers, they wanted real carriers, but couldn't hope to get such ships approved politically, so they put in as much carrier capacity as they think they could get away with, but seemed to have almost deliberately hamstrung the design in many important ways to get the design approved.
There is a reason the most technologically and financially capable navy on earth does not bother with such niche designs even though the primary threat from their biggest adversary was underwater.
Specialist ASW carriers are unnecessary. You just end up wasting money and space adding capacities you never want such high value assets to have to use onto ships that should be as far from the action as you could manage.
You may even require your CVNs carry contact minesweeping, describing it as "specific needs".
That's just an absurd strawman not worthy of further comment.
Problem is cost&size, apart from somewhat contradicting requirements. For example, hull lines and shape(both below and above the waterline), propulsion plant(not just power; again, compare typical modes of operation), internal subdivision, hangars, etc etc.
Again, if you stop using the Japanese ships as the benchmark and critically assess what capabilities you really need from a ship directing ASW from afar instead to getting stuck into the thick of the fighting (where you do not want such high value assets), just what requirements other than speed, hanger and deckspace does a ship need to function as an ASW carrier? And how does that clash with the amphibious assault capabilities of an LHD?
It is perfectly reasonable to roll those two roles into one, especially in a Taiwan scenario, since those LHDs could also conduct air assaults against targets on Taiwan.
LCS is more than 2 times heavier than 056, and is closer to 054A in size, if anything.
In areas other than size they have even less in common.
LCS are designed to fight in someone else's backward, whereas the 056 is designed to fight in China's own backyard.
But the small size of the 056s lends it even more advantages in ASW, because the PLAN can afford to use them in greater numbers to provide much more redundancy and extra coverage, and they are considered too small a target to be worth engaging for enemy subs. Especially since they will know that there are several other 056s close by enough to quickly respond to any such attack.
