As the title says.
This mechanics makes not sense and I think a new approach is needed here.
We can discuss what that approach may be but please refrain from stating stuff like "no, will be abused and it has always been that way".
Instead of getting simply deleted the units should be allowed to find a new home, in my humble opinion. If you think that makes a it too easy to kill a world wonder, then maybe we should think how to adjust for that.