US needs Marine core because they project power through sea and air. Hence the strongest Navy and the second strongest air force in the world. In addition to this they have Islands to protect: Guam, Diego Garica, Hawai, Japanese Island Chain, Philippines etc. Combine it with 10 aircraft carriers with 5k+ troops which are an island on their own. It serves their purpose, the same is not true for us right now maybe 20 years later.
We have Andaman, Lakshadeep, Maldives etc. In the future maybe seychelles, madagasacr, Mauritius, Oman.
Of course MIC is the key here u don't want to build your Marine corps with imported LHD, helis and ships.
The 800 billion that America spends is invested in their own economy, its basically like MNREGA to them except with positive balance sheet.