Not sure if I get your point but I think it is this: Since we have not researched the curtailment of all fossil fuel sources with green sources, we cannot proceed with replacing fossil fuels with green energy. Correct?
Your argument says that since we don't know, we can't proceed. Yet we do know that replacing some fossil fuel sources with green sources reduces the impact on the environment. 
Since we can't ever know for sure what will happen when renewables replace ALL non-renewables, is that a logical argument for not doing anything? I agree population is the key, but how long will it take to cut the world's people by say half? And can't we apply your argument that since we don't know for sure what will happen, we can't proceed?
The argument you put forth is the fallacy of complexity: since we don't know something that is complex, we can't do anything.