For example, the Arleigh Burkes are close enough to the same weight as a Ticonderoga class cruiser.

I guess some of it could be the nitpicking of what's a "heavy destroyer" versus a "light cruiser".

In planning for Operation Barbarossa, the Germans planned for the limit of their advance to be the A-A line, roughly from Arkhangelsk to Astrakhan. While part of this line was defended by the Volga River, much of this line was not anchored on any geographic features of significance

However, "just" a few hundred kilometers further East was such a feature that would cover almost the entire front, with the exception of the semi-desert of the Kazak steppe - the Ural Mountains. An eventual line based on the Urals would also capture significant Soviet industrial centers, and put many others within bomber range

Obviously none of this ended up mattering - but why was the furthest limit of the Nazi advance expected to be the fairly arbitrary A-A line, vs the Urals?

I understand why normal infantry in dedicated ships took the lead, amphibious assaults became a lot larger and needed a lot more specialised equipment, while sailor landing parties couldn't really sustain themselves for more than a few days on land and lacked heavy equipment.

but what was the downside in having a few extra battalions or regiments of infantry to help with the first few days of a landing when integrated within the combined arms systems of larger army/marine dominated landing force that meant they seemingly very suddenly just stopped being a thing?

Since the 41st was the only Royal Marine that fought during the Korean War, where they raided the North Korean coast with the Republic of Korea Marine Corps, I wonder what prevented the ROKMC forming a unit similar to the commandos?

I do know that they were base on the USMC but surely there would have been an attempt.

It is probably both but which influences the other one more substantially? For example in WW2 trench warfare is much less dominant than WW1, this is a result of developments in mechanised forces with radios, airborne forces, and artillery. However are these advancements made with the goal of counteracting trench warfare in mind, or are they natural results of increased productivity and scientific findings which caused trench warfare to be outdated?