Evita, Michael was saying that the reporters on the Corleones' payroll would love a story about how a dead, crooked Mac (killed by Michael) got what was coming to him. I'm saying that they'd love even more to have the opportunity to expose a live, crooked cop mixed up with drugs and murder because, when the Police Commissioner fired, suspended or transferred Mac, they could brag that they did a "civic duty" by "exposing corruption."
But Sollozzo was the problem, not McCluskey, McCluskey was just collateral damage.
Michael's whole point was that they can't wait, because Sollozzo was going to kill Vito ("That's the key for him.")
How long would it take to get McCluskey fired, transferred or suspended? Days? Weeks? Months? Even once he was dead, it seemed to take a while for the newspapers to get enough out on McCluskey to allow things to loosen up. It surely would take longer if McCluskey was still alive.
And getting McCluskey out of the picture doesn't mean that Sollozzo is no longer a threat to Vito. It just makes Sollozzo a bit easier to kill.
