I have served with both the Army and Marines where we supported both. And I will tell you this. There is a huge difference in missions between the Army and the Marines. The short of it is, if you want it done right now using a scalpel, you call a marine. If you want it done hard and fast where it doesn't have to be pretty but it has to get done by a large force,you call on the Army. An old joke comes to mind
There have even been studies back in the 1950's into disbanding the Corps. And the Army basically shook their head and said they could not do it.
And that was finally laid to rest during the Korean War.
There are huge differences between the two branches. The Army rarely operates at any level below Brigade or Division strength. Tens of thousands involved in operations, with massive formations and a logistics train to match.
The Corps however very often operates with single Battalions. Tell the Army they are sending out a single Battalion to run an operation and they will likely tell you to go pound sand, they are not going to do it. They operate at the level of Brigade, Division, Corps, and Army. Not at a Battalion level. They do not train that way, they do not operate that way. That is why in WWII they were sent to Europe, where hundreds of thousands were needed to bash their way into Europe.
When the mission was to take islands, they used the Marines.
And it is like that even today. When the Marines were sent to Lebanon to support UN forces in 1982, they sent a Battalion of Marines. Specifically BLT 2-8. In other words, the 2nd Battalion, 8th Marine Regiment, 2nd Marine Division. Who then in 1983 were relieved by BLT 1-8. They were the ones there when the suicide bomber drove a truck in and killed over 300 Marines and Sailors.
Even the Army realized that they would have to change radically if they were to even try to take on the duties of the Marines. And in essence, create entire Divisions that were almost completely independent of the other Infantry Divisions of the Army. Specifically, to learn and operate as Naval Infantry, doing amphibious landings as well as helicopter operations. As well as operating aboard ships for months at a time and not just using them as a taxi as they always had in the past.
Tell a Regiment of Soldiers they are getting on an LHA, LPH or LSD and they are going to be living there for the next 6+ months, living stacked 3-5 high in a sleeping area no bigger than a cot and the morale and other issues would be huge. The Army has never really operated with the Navy, not in over 200 years. Even in WWII, far more were hauled around by the Merchant Marine than were moved by the Navy.
And they literally do not even speak the same language, or have traditions anywhere close to each other.
Then, there is the Air Force. They would fight it also, as there is no way they are letting the Army ever get their hands on Marine Aviation. And the Navy sure as hell does not want it, like the Air Force they want supersonic fighters to go screaming at ships or enemy air bases. Not "low and slow" aircraft with the primary mission of air to mud close air support.
I also have served with both, in fact over a decade in both. 10 years as a Marine grunt, and 14 years in the Army. And the two are vastly different. They have never operated in the same way.