That would depend. Many militaries also feature commando units that proportionatly number closer to the marines and basically consist of the upper tier shock troops. It’s a very similar structure.
This has not been the experience within the military structure in practice. Sharing sounds nice until these conflicts are raised. In those events, the military has seen fit to designate assets with differentiated priorities and command chains to avoid breakdowns. I think you believe the Marine air wing is large enough to single handedly conduct an offensive and thus make it redundant. It is not. However, it is large enough that given a created conflict as such, Marines would have sufficient air support in an emergency and the fleet would have sufficient air cover as well. In addition, this allows the marines to move some planes off ship with them. Basically this allows streamlined intelligence and command and very little redundancy because it shares support units at a favorable marginal cost. Think of it as taking a general asset of a fleet of aircraft and dividing priorities by putting them under different units. Some have 1) Fleet defence 2) supporting land operations while others have 1) Supporting Marine operations 2) Fleet defense and the units are split up based upon expected needs in a conflict. While support is shared, you may ask what’s the point. By speaking with marine officers I’ve known, and reading things like their Warfighting Manual, they outline pretty clearly that the marine mandate is to be able to conduct maneuver warefare (the army does not share this capablility and instead focuses on attrition warefare), as required in most invasions. Due to the speed and flexibility required in these offensive operations, it makes a lot of sense to streamline command and have things centralized under the Marines. I’ve heard repeatedly that you can not underestimate the amount of speed that is lost through decentralized chains of command. Beneath a certain scale, a centralized command makes sense (why we have the marines) and on the larger European theater style conflicts, segregation as seen in the army and air force begins to make sense. That is the other reason for unit segregation. I have a feeling the Pentagon has a reasonable amount of expertise here, and they seem to share the same view.
At the end of the day, warefare generally falls under maneuver warfare and attrition warfare. In the past decades, there’s been a rise in prominance and effectiveness of maneuver warfare, while in the future they may be a need to return to an attrition style of fighting. The army and the rest of the military is designed for the latter while the marines are designed for the former through structure, training, philosophy and equipment. I don’t think the need to be able to wage both manners of warfare given the variety of conflicts we get involved with should be underplayed. This is again a distinction that most people don’t seem to be aware of.