Yeah, but it's nonsense. They have no admissible evidence against Marlo.
On the old bodies, no. But the only way Marlo is going to stay out of trouble is to keep out of the game completely, and his last scene implied that won't be possible for him, it's all he knows. The good police of Baltimore will never forget about him, so if he gets back in it will just be a matter of time.
Yeah, but Marlo was difficult to get up on in the first place. He was mostly low tech so a long-term operation was difficult. Plus, the city didn't want to pay for a long-term operation.
The Wire had a message, and that message is that things are messed up, and they'll continue to be messed up for a long time. Killing Marlo would provide the audience with a false sense of victory, a false hope that the world is just, and a false impression that five years of fighting those gangs was making Baltimore a better place. That's not the story that The Wire was trying to tell.