They aren't there to help put a company back on track. Not one McKinsey powerpoint in god's existence has resulted in someone gaining a job, or a layoff being avoided.
McKinsey is a Sin Eater (read up on the concept). Comcast leadership already knew they wanted to cut labor. It's the easiest target on every company's P&L statement that executives can cut, sell as "efficiency" or "cost savings" and because Wall Street is incredibly stupid and gullible, it will slightly bump the stock (Comcast stock has always been trash for the record).
This isn't about the company, or economics directly; a McKinsey report always serves only one purpose - it allows failed executive leadership to go to the board of directors, point to the report and say "I have no choices, there are no other alternatives, even McKinsey says so; we have to do this and it's not my fault we're here." Now the executives take no accountability as McKinsey said they have to do the layoffs, they get to stay on or keep their golden parachute on the way out - only to go do the same damned thing at the next company. McKinsey keeps being brought into the same company every few years for the same exact reason, execs need air cover to make cuts to cover up their sc--w-ups. No senior executive will ever admit to making errors or being wrong.
Also remember, most board of directors are the same executives running this play every few years in other companies. Board members sit on Comcast's board, but have jobs elsewhere double dipping for a salary. The gentleman's agreement is I'll let you slit your employee's throats so long as when I ask you allow me the same.
100% what @dx+1m3teyh86 said.