MACOs on ENT should have logically made Malcolm redundant
Not really. It would relieve Malcolm of his security role, but he was still tactical officer. A lot of naval ships had marines on board to serve security roles while weapons maintenance and operation would be performed by a different group of naval officers and seamen.
I feel like that also drove a lot of colonization for humanity. It doesn’t make sense that so many people would want to leave Earth if it became paradise unless it wasn’t as fulfilling and you could be in charge of more if you went off world.