Numbers 33:50 And the Lord spoke to Moses in the plains of Moab by the Jordan at Jericho, saying, 51 “Speak to the people of Israel and say to them, When you pass over the Jordan into the land of Canaan, 52 then you shall drive out all the inhabitants of the land from before you and destroy all their figured stones and destroy all their metal images and demolish all their high places. 53 And you shall take possession of the land and settle in it, for I have given the land to you to possess it.... 55 But if you do not drive out the inhabitants of the land from before you, then those of them whom you let remain shall be as barbs in your eyes and thorns in your sides, and they shall trouble you in the land where you dwell.
Now that the people were prepared to enter the land, God reminded them of the big picture: He was giving them this land. Those currently in the land had completely rebelled against Him and were not turning back. Coexistence would not work. Syncretic religion does not work. Even a little leaven of sin would spread and reinfect the land and their lives. To not clean house and start afresh would be asking for trouble. How much trouble do we make for ourselves by partial obedience, selective hearing, and thinking we know better than God?