Also would need to position in a number of crews from Addis Ababa to handle the flights, so an ability to reinstate them has probably gone now. What doesn't add up is the trips were sold some weeks ago but the slots were only applied for and then denied last Friday. Ethiopian don't normally serve Kiev but I would hope they would understand how to get slots there earlier than that.
There have been various charters using long daystop aircraft over time. Qantas used to do 747 cruise charters from London to Athens on a similar basis.