We can demonstrate gravity by dropping a ball. We have demonstrated lack of gravity with space travel. We have simulated lack of gravity using airplanes and pools. It is a theory which has been proven beyond a reasonable doubt.
We have no way to simulate GW on a large enough scale and with enough variables to prove or disprove MMGW. The fact that the earth has been MUCH hotter and MUCH colder at several times in the past without man suggests that nature is far more powerful than man which to me makes it less likely that GW is man made or significantly impacted by man.
And when you form a theory and make predictions based on that theory and you don't get the results you expected, you don't just change the theory to match the results.