To eliminate the evil cult followers in the wasteland and the "sentinel" behind them is the truly complete measure, which Gawain is certainly aware of.
But those evil cult followers' intentions are quite obvious: they know the vast and perilous Gondor wasteland serves as a safe barrier for them. If the armies of the mortal world were to fight within the wasteland, they would be at a severe disadvantage, and under this protective barrier, they are determined to cultivate their plan to fruition. They won't initiate any action until their "grand plan" is complete.
Currently, Cecil's "Project 115" is not yet finished, the Typhon Empire's Mage Corps is still gathering, and the expeditionary armies from the Silver Empire are not ready yet. Before the most powerful military forces in the mortal world are prepared, will those evil cult followers complete their plan first? No one can say for certain.
