As to your last point, since when does the MSM have fear of sounding like jackasses when cheerleading for a lefty candidate and/or knocking a relatively conservative one? They have absolutely no shame.
With regard to the Weiner and Spitzer foibles, those would be ancient history by the time the general election rolls around in November (from their vantage point), as might be some or all of the other stuff you mentioned.
So if the GOP candidate Lhota issues any statement warning of an impending disaster should NYC return to the bad old pre-Giuliani days (i.e., the Dinkins regime), there's no reason that the MSM couldn't play the race card against him, especially now when then Dem candidate will be either a black man or a white guy married to a black wife with two black kids who try to look and act black.
But there are very good reasons. When Weiner, Spitzer, etc., are ancient history, so will the slogan be. You can’t have a feeding frenzy over leftovers. The MSM often does make a jackass of itself, which is why they have an aversion to it, and choose their targets with care, and if it doesn’t take off right away, they drop it. Don’t fool yourself that they are idiots. They are when it comes to dictating public policy, understanding complex situations such as the Middle East, the environment, and the criminal justice system. But that’s not their area of expertise. They are experts and even geniuses of a sort when it comes to manipulating the public. My proof, the fact that they do it so often, and make so much money at it, despite being idiots on the very subjects concerning which they are manipulating the public. So the statement as I formulate it will not lead to a feeding frenzy. It doesn’t mention Dinkins by name, and could just as well refer to Ed Koch, Abe Beam or John Lindsay. So there goes the race card. By the time they make the case for the race card, people will no longer remember what they’re talking about, and they’ll be on to Prince Edward potty training his little heir apparent too soon, or a zoo animal giving birth, or some other MSM-type thing.