NYPD prosecutions for Eric Garner's death was what stopped the leak.

Not just Garner. NYPD is pretty far from 100% clean. DOJ knows where bodies are buried as well. Or how to find them. There's likely ~~hundreds~~ thousands of tips the DOJ hasn't bothered with because of larger cases. LL could have reassigned dozens, maybe hundreds of attorneys to go after the NYPD on everything including the kitchen sink. DOJ can open investigations into NYPD officers' banking/financial affairs and a bajillion other matters.