This might be something they’d try, but it wouldn’t work. First of all, everything in the current cycle is overshadowed by Anthony’s Weiner flipping off reporters, and Spitzer the sociopath losing his revival bid, Syria and the senior swimmer lady, Prince Edward’s reality show, and other stupid stuff. The LSM can’t just conjure up a feeding frenzy at will; all the stars and planets must align for a feeding frenzy to happen, and the sharks are stuffed on too much junk food to get up an appetite at present. Second, the perpetually offended would have scant material to go on. They do want to go back to the days before Rudy, and they will vote Democratic. I think they’ll predict that the slogan will backfire, and snigger oh-so-smugly, ‘til the election results start coming in. There’s not enough there to translate into sufficient angry chimp noises (there, now that’s a phrase that they’d get upset about, if DiBlasio had used it). There’s also the fear of sounding like a jack-ass, which is what happens when you try to make a feeding frenzy out of a molehill and the sharks aren’t biting it.
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.