Drogon is the last remaining dragon. In the fifth episode he was used by Daenerys Targaryen as a weapon in destroying of King's Landing. After the death of the Night King and burning of all the Scorpion weapons nobody has any chance to kill the last dragon.
With Daenerys Targaryen taking a major turn into Mad Queen territory, her fate may be sealed, but there is still no reason for killing Drogon. John, as a true Targaryen, who can ride and communicate with dragons and as the new King of Westeros could keep Drogon around.