There is periodic talk of dismembering the USMC under the rational that it's redundant. But it will not happen for a number of reasons:

Tradition: The USMC dates back to 1775, as does the Army and Navy.

Patronage: The Marines seem to have friends in high places (i.e. Congress).

Money: Typically Marine operations are cheaper.

Patience: Who else would sit on a boat (okay ship) for weeks upon weeks off shore just waiting to see if it's going to be an amphibious landing or just another NEO.

Heritage: Marines suck it up (dumbest thing IMO the Army ever did was to get rid of the regimental system).

Esprit-d'corps: Marines love being Marines, and they have the best uniforms!