The Marines are an amphibious fighting force. If you eliminate the Marines and replace them with Army forces, how are you saving money? You still need the same number of MEUs, or whatever the Army equivalent would be.
When's the last time we HAD to make an amphibious landing somewhere? How much are you willing to spend to maintain a force for a mission which isn't called for anymore?
And, by the way, the Army is perfectly capable of conducting amphibious operations. In fact, they've done it more than the Marines anyhow, including the largest and arguably the most successful in history at Normandy.
Quite a bit could be saved by closing down Marine bases here at home and abroad. Without a Marine Corps, we wouldn't need Pendleton or Twenty-Nine Palms or Barstow or PI.
That all sounds so easy and pat.
Not so-I am afraid you require massive injections of history and massive doses of subject matter as to the operational art(s) of warfare.
Building, supplying and sending off an invasion force from 30-40 miles away (Portsmouth, Southampton, Chichester) where in you have a solid, fully protected logistical post, unsinkable airfields ( that don't need fuel and don't move) providing virtually instantaneous, deadly and un- interfered tactical air support ( plus massive strategic sppt. as in bombing enemy rail-heads, supply dumps etc.) is very much different than say Guadalcanal, Tarawa, Peleliu, Iwo Jima etc.
You'll notice I didn't include Okinawa, if you understand the reason why I didn't then you might now understand why Marines and soldiers are different and why the distinction is wise. ( in addition if you knew some history of the US Armys 7th & 25th or 81st divisions that would help too

).