I don't think the serial killer plot would work if it hadn't been for the effort setting it up in the previous four seasons. You have to see both McNulty's battle with his personal demons (and descent back into them), and his frustration with the ineptitude of the Baltimore PD's bureaucracy, to believe he would be fed up enough (and off his rocker enough) to try this tactic when the PD has no more money. You have to see how City Hall's political gamesmanship works, and you have to see how Tommy Carcetti is as a person, to see that his ego and ambition are too big to allow him to take the state money, so the decision to defund the police department makes perfect sense as a character choice in this situation.

Whether or not the journalism side works for you is debatable. It's certainly true that none of the characters are particularly well-rounded and that they function more as plot devices. But at this point of the show I find it perfectly believable that one reporter would start lying in an effort to boost his career and win awards, and perfectly believable that two managers would let it go because they care more about winning awards than reporting facts. Even though I don't know much about the journalistic world, I believe it because it's the way every institution works in The Wire's world, and I've seen four seasons of gangs, drugs, business, politics, and the school system work in the same way.

That's because those guys were similar, so it's who they became

If you look at Omar, he would never kill someone that wasn't in the drug rings, he wouldn't curse, etc. There's that scene when he robs the stash in the little corner store, and then pays for the Newports that he buys. Then you like at Michael, and he's always questioning Snoop and Chris about why does Walter have to die, or why someone else has to. When he's watching the back of the house, the little kid runs out, and he does nothing to stop him.

Then you look at Dukie and Bubbles. Bubbles is a guy who lives on the street, but people showed compassion towards him (Greggs, the white guy whose name I forget) and he showed it back. He tries to teach Sherrod math and even wants him back in school. Dukie is a smart kid who figures out the computer in class, wasn't built to be a part of the game, and who some show compassion for. His first job is running the scam with the guy from the stables on aluminum, the same thing which Bubbles did.

It's not that they became Omar Jr and Bubbles Jr, but they became a mirror image of the type of person. When those two grow old, there will be kids just like them who fill their role. It's all in the game

Sydnor becoming McNulty makes sense too. I forget which episode, but McNulty is drunk with Bunk talking about the real police and the problem with it all. He mentions Sydnor as one of the real police in the entire unit.He realizes what they did as Major Crimes was real police work (to quote Freamon) and made an impact. When the unit was made into two people, he was there with the judge and Rhonda, so he had that connection.