A force of smaller ships simply do not have the firepower, the command and control capabilities, and general persistence to match a force of larger ships.
More importantly, a force that includes a variety of warships will allow them to complement each other. A carrier can provide CAP and air borne early warning for a task force. Large destroyers can provide more capable air defence radars and command capabilities over smaller destroyers and frigates, while also having more firepower as well. LHAs can provide a dedicated platform to operate from.
A combined arms unit is almost always superior to sending a force of only one type into the fray and it applies for naval warfare as well. You want your different ships to complement and assist each other.
If you really think it is a good idea to exchange 3-4 054As for a single aegis destroyer or cruiser then I must say you are being ridiculous. That kind of kill/loss rate would be unacceptable.
You ask why the PLAN would need larger ships to fight a local/westpac naval war. My question to you, is why do you think the PLAN wouldn't need such ships? Asymmetrical warfare will only take you so far, and the PLAN have reached a point where their main offensive and defensive capabilities do not lie only in asymmetrical capabilities like mines and FACs. Asymmetrical naval warfare will allow you to harass the enemy, and defend close to shore. but it won't allow you to take the battle beyond green water, which is what the PLAN is now aiming to do. Sending a massive force of only 054As against a combined opposing force made up of aegis destroyers and carriers will be suicide.
And you are very mistaken to think that the PLAN's large vessels are only for prestige. The capabilities that 055, 052C/D, and carriers all bring are vital and survivable if used in the right way. The capabilities they bring are also more important when engaging a higher tier navy like USN and JMSDF than if you're engaging a second tier navy like Vietnam's.
You seem to believe that any PLAN force that goes against JMSDF or USN will be bound to suffer massive casualties, therefore it is wise to build a large number of less capable ships. But if your massive fleet of 054As are inherently limited by their SAMs, and can't get anywhere near within firing range of enemy ships because they only have YJ-83s, then the enemy can simply keep out of range from your ship's air defences and AShMs and send fighters loaded with AShMs from carriers. And you have no long range air defence SAMs and more powerful radar to defend yourself with, you have no AEW or CAP because you have no idea. In other words, if you only have one type of vessel with limited capabilities, it won't matter how many you have, because your enemy has other vessels that directly counter your frigates.
There are some instances where massing only a single type of asset is useful, but if the PLAN want to take on USN and JMSDF, they need a combined arms approach.
Comparing Yamato/Musashi to the PLAN's 052C/Ds, 055s, and future carriers is also a mistake, because Yamato/Musashi were made irrelevant and vulnerable due IJN lacking have enough supporting capital ships to leverage the strengths of their large battleships near the end of the war. More fundamentally, aircraft carriers made large battleships obsolete as the primary offensive arm of naval taskforces, however in the modern day and into the near future, large destroyers and carriers are still the essential offensive and defensive assets of navies.
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You also seem under the impression that 052C/Ds and 055s are only going to be buitl in limited numbers. Remember, 6 052Cs have been produced and we are expecting 12 052Ds as well. That is 18 052C/Ds in total. Not a small number. We don't know how many 055s will be produced, but expectations run into the double digits. I do not expect it to be only a 3 ship affair like Zumwalt. And remember, current PLAN destroyer flotilla arrangement has about the same number of destroyers and frigates, so once the PLAN is full modernized we will likely have an equal number of modern blue water destroyers as well as frigates. That should demonstrate to you that ships like 052C/D and 055s are not going to be lacking in number.