When the largest airport in Northern England refuses all diversions, do you realise the pressure this places on ATC, amongst others? Other agencies have to deal with the fallout. They don't have the option to shrug their shoulders and look the other way. And they aren't overrun with extra staff on call either.
Declaring your unavailability in advance should actually help ATC and the airlines - crews won't plan for MAN as an alternate. What is worse is saying your available but then having to turn flights away at the last minute, creating additional workload for crews and ATC replanning.
Running short of ground crew because of flights not running to schedule is nothing new - I remember running around like a blue a*sed fly 40 years when everything turned to sh*t, as my supervisor used to so eloquently put it. It's not just about good management, somebody has to pay for the system to be running with sufficient flex to cope with additional flights. BTW, the main restraining factor for us taking diversions years ago used to be availablity of coaches for onward transport...