The cost to either country is irrelevant if a head of state wants to take a holiday abroad with their family - that would be a choice and judgment he and the other country makes between them.
Hasn’t held him (or Camilla more recently)!taking private holidays abroad - who they see there isn’t relevant.
KC3 might want to do it (if well enough) to call MM’s bluff?
To see his DGC in his lifetime?
To put the DC first if their own parents are obtuse?
Often it takes the actions of a wise older person to shine a light on the right path to family healing.