If Justice were eliminated, you would not have a federal criminal investigative force capable of dealing with crimes, corruption, and offenses that cross state lines. A cop from Texas cannot arrest someone in New Jersey, he has no jurisdiction. I think that is the biggest thing. After all, under Justice you have the FBI, US Marshals, DEA, ATF, Inspector General, all the civil rights enforcement, and a boatload of others.
Could you justify eliminating the department as a whole? I do not think so. Law enforcement is one of the fundamental duties of government. I do think a lot of things could be consolidated. DEA, ATF, and FBI could all be combined into one organization, for example. It appears to me that several offices overlap their duties and could probably be consolidated. But as with everything, the devil is in the details.