The problem I see with Manston as a passenger airport is that it's too far from London to be marketed as a 'London' airport but close enough that London airports are easily accessible to the population of Kent.
As for being a cargo hub in a Brexit era, not every type of cargo is suitable for air transport.
I wish them well, but my suspicion is that after a year or two the new owners will declare the place unviable for aviation use and try and develop it in different ways......