Firstly, the RAF has been cutting it's display budget for years and I cannot see any chance of the memorial flight being expanded.
My guess is that the machine would only be able to display for a limited number of hours each year and only at/over specified sites due noise and safety. A corporate will want their billboard to be available all year round and at many venues. For example, if a corporate sponsored the repair of 50 vintage cars (trying to find equivalent sum of money) then they can parade those cars at any racing ground in the country or any city square that they choose. XH558 cannot be made to do that.
Yes of course the machine has unique attractions but I cannot imagine a corporate even agreeing to see the folks for half an hour. There are so many offers for places to stick their names that they can pick and choose and would not choose one that has narrow appeal and narrow visibility. Lastly, in what ways does a Cold War bomber link with 21st century corporate values? (OK, enough jokes about Halliburton already)
It's just that there is nothing that a modern company want from a machine that is old and noisy (they will say) and can no longer do some of it's most startling things. It's speed and performance are limited and were only ever done at high level. If they cannot find a pure enthusiast multi-millionaire, then it is over. Since any pure enthusiast multi-millionaire would have already been part of the gang for years ... it is over.