Both Feanors idea and King of Towns criticism have some appeal...
There were examples in the history, where "captured" (or surrenedered or sometimes defected) "Great People" were used - the one that comes to my mind first is Wernher von Braun and the whole Nazi-Rocket-Science-Crew which were taken by / defected to the USA and were crucial for the US Space program. (and others - less prominent but nevertheless important, did the same for the Soviejets)
Perhaps introducing some randomness - like a 50% chance to Kill/Take would be a solution... Or maybe making a new Version of the GP, when captured, who only have some, but not all abilities - lets say can bulb techs and finish buildings, but can not be settled and not used for Golden Age...